Grub supports JFS to some degree; systems boot on JFS over multipath with relative ease (see below). Using JFS without multipath, it seems to fairly reliably fail to complete the install (so of course you wouldn't manage to boot). In this case it looks as though detection of jfs fails in grub-install.
I did run into a kernel crash though, while booting JFS-over-multipath. Furthermore, JFS in general seems to have issues with filesystem corruption here. From one boot to the next, the system may drop to the initramfs failing to mount the root partition (with an "Invalid argument" error; running fsck.jfs seems to fix the issue). Kernel oops for the crash I got follows:
Grub supports JFS to some degree; systems boot on JFS over multipath with relative ease (see below). Using JFS without multipath, it seems to fairly reliably fail to complete the install (so of course you wouldn't manage to boot). In this case it looks as though detection of jfs fails in grub-install.
I did run into a kernel crash though, while booting JFS-over-multipath. Furthermore, JFS in general seems to have issues with filesystem corruption here. From one boot to the next, the system may drop to the initramfs failing to mount the root partition (with an "Invalid argument" error; running fsck.jfs seems to fix the issue). Kernel oops for the crash I got follows:
[ 6.626202] Unable to handle kernel paging request for data at address 0x00000000 ME,IR,DR, RI,LE> CR: 44048824 XER: 00000000 0x1dc/0x650 [jfs] 0x178/0x650 [jfs] 0x178/0x650 [jfs] (unreliable) 0x4c0/0xa20 [jfs] 0x88/0x100 [jfs] real+0x64/ 0xe0 0xcc/0x3c0 open+0xfc/ 0x170 execat+ 0x9c/0x2c0 common. isra.14+ 0x2b8/0x8c0 0x5c/0x80 call+0x38/ 0xb4 ME,IR,DR, RI,LE> CR: 48008824 XER: 00000000 0x1dc/0x650 [jfs] 0x178/0x650 [jfs] 0x178/0x650 [jfs] (unreliable) 0x4c0/0xa20 [jfs] 0x88/0x100 [jfs] real+0x64/ 0xe0 hash+0x60/ 0x80 0x310/0x420 0xb8/0x1d0 lookup+ 0xa8/0x1b0 0x50/0xf0 0x34/0x60 call+0x38/ 0xb4
[ 6.626322] Faulting instruction address: 0xd000000003f3e1cc
[ 6.626366] Oops: Kernel access of bad area, sig: 11 [#1]
[ 6.626492] SMP NR_CPUS=2048 NUMA pSeries
[ 6.626536] Modules linked in: autofs4 jfs dm_round_robin ibmvscsi ibmveth scsi_dh_emc scsi_dh_rdac scsi_dh_alua dm_multipath
[ 6.626685] CPU: 4 PID: 2176 Comm: systemd-udevd Not tainted 4.4.0-92-generic #115-Ubuntu
[ 6.626771] task: c000000004e82a20 ti: c000000004f08000 task.ti: c000000004f08000
[ 6.626823] NIP: d000000003f3e1cc LR: d000000003f3e168 CTR: c0000000001195c0
[ 6.626873] REGS: c000000004f0b470 TRAP: 0300 Not tainted (4.4.0-92-generic)
[ 6.626921] MSR: 8000000100009033 <SF,EE,
[ 6.627110] CFAR: c0000000000108e8 DAR: 0000000000000000 DSISR: 40000000 SOFTE: 1
GPR00: d000000003f3e168 c000000004f0b6f0 d000000003f590d0 c000000770e1b440
GPR04: d000000003f4fba0 d000000003f52e58 00000000000007e1 0000000000000001
GPR08: 0000000000000000 0000000000000000 0000000000250003 d000000003f4aad8
GPR12: c0000000001195c0 c00000000fb42600 fffffffffffffffb 000001003b9b18d0
GPR16: c00000077ba92d60 c00000077ba92c48 000001003b9b0080 c00000077ba92e88
GPR20: c000000004f0b9e0 c0000000030789fe c000000778fda000 c000000004f0b900
GPR24: 0000000000250003 0000000000001000 0000000000000000 d000000003f52e38
GPR28: 0000000000250003 f000000001dcb200 c0000007752a7b00 c000000770e1b400
[ 6.628023] NIP [d000000003f3e1cc] __get_metapage+
[ 6.628065] LR [d000000003f3e168] __get_metapage+
[ 6.628108] Call Trace:
[ 6.628127] [c000000004f0b6f0] [d000000003f3e168] __get_metapage+
[ 6.628193] [c000000004f0b7d0] [d000000003f394e0] dtSearch+
[ 6.628247] [c000000004f0b8e0] [d000000003f23b68] jfs_lookup+
[ 6.628314] [c000000004f0ba10] [c0000000002f3864] lookup_
[ 6.628408] [c000000004f0ba50] [c0000000002fa324] do_last+0x9f4/0xff0
[ 6.628455] Unable to handle kernel paging request for data at address 0x00000000
[ 6.628458] Faulting instruction address: 0xd000000003f3e1cc
[ 6.628641] [c000000004f0bb10] [c0000000002fab3c] path_openat+
[ 6.628707] [c000000004f0bb90] [c0000000002fca5c] do_filp_
[ 6.628771] [c000000004f0bcb0] [c0000000002ed82c] do_open_
[ 6.628833] [c000000004f0bd20] [c0000000002f0148] do_execveat_
[ 6.628912] [c000000004f0bdf0] [c0000000002f0b2c] SyS_execve+
[ 6.628977] [c000000004f0be30] [c000000000009204] system_
[ 6.629038] Instruction dump:
[ 6.629070] f91f0020 f93f0028 fbdf0030 fb9f0038 e93d0000 7927afe3 418203fc ebdd0030
[ 6.629197] 41920034 e93d0008 e95f0038 811e0000 <e9290000> 39080001 81290090 911e0000
[ 6.629313] ---[ end trace fd44b94117cd8463 ]---
[ 6.631101]
[ 6.631129] Oops: Kernel access of bad area, sig: 11 [#2]
[ 6.631199] SMP NR_CPUS=2048 NUMA pSeries
[ 6.631238] Modules linked in: autofs4 jfs dm_round_robin ibmvscsi ibmveth scsi_dh_emc scsi_dh_rdac scsi_dh_alua dm_multipath
[ 6.631391] CPU: 7 PID: 2173 Comm: dmsetup Tainted: G D 4.4.0-92-generic #115-Ubuntu
[ 6.631481] task: c0000000045a5440 ti: c00000000466c000 task.ti: c00000000466c000
[ 6.631554] NIP: d000000003f3e1cc LR: d000000003f3e168 CTR: c0000000001195c0
[ 6.631625] REGS: c00000000466f510 TRAP: 0300 Tainted: G D (4.4.0-92-generic)
[ 6.631678] MSR: 8000000100009033 <SF,EE,
[ 6.631806] CFAR: c0000000000108e8 DAR: 0000000000000000 DSISR: 40000000 SOFTE: 1
GPR00: d000000003f3e168 c00000000466f790 d000000003f590d0 c00000076e340040
GPR04: d000000003f4fba0 d000000003f52e58 0000000000000b34 0000000000000001
GPR08: 0000000000000000 0000000000000000 0000000000020090 d000000003f4aad8
GPR12: c0000000001195c0 c00000000fb44280 0000000000000007 0000000000000000
GPR16: c00000077ba919e0 c00000077ba918c8 0000000000000000 c00000077ba91b08
GPR20: c00000000466fa80 c0000000036587fe c000000778fda000 c00000000466f9a0
GPR24: 0000000000020090 0000000000001000 0000000000000000 d000000003f52e38
GPR28: 0000000000020090 f000000001dc4700 c000000004940cc0 c00000076e340000
[ 6.632836] NIP [d000000003f3e1cc] __get_metapage+
[ 6.632933] LR [d000000003f3e168] __get_metapage+
[ 6.633009] Call Trace:
[ 6.633078] [c00000000466f790] [d000000003f3e168] __get_metapage+
[ 6.633199] [c00000000466f870] [d000000003f394e0] dtSearch+
[ 6.633329] [c00000000466f980] [d000000003f23b68] jfs_lookup+
[ 6.633458] [c00000000466fab0] [c0000000002f3864] lookup_
[ 6.633615] [c00000000466faf0] [c0000000002f8630] __lookup_
[ 6.633693] [c00000000466fb40] [c0000000002f8960] walk_component+
[ 6.633771] [c00000000466fbb0] [c0000000002f9648] path_lookupat+
[ 6.633825] [c00000000466fc00] [c0000000002fba18] filename_
[ 6.633879] [c00000000466fd20] [c000000000332ee0] user_statfs+
[ 6.633930] [c00000000466fd70] [c000000000333054] SyS_statfs+
[ 6.633992] [c00000000466fe30] [c000000000009204] system_
[ 6.634067] Instruction dump:
[ 6.634106] f91f0020 f93f0028 fbdf0030 fb9f0038 e93d0000 7927afe3 418203fc ebdd0030
[ 6.634233] 41920034 e93d0008 e95f0038 811e0000 <e9290000> 39080001 81290090 911e0000
[ 6.634363] ---[ end trace fd44b94117cd8464 ]---
[ 6.635762]