I have tested on Trusty with "preseed - 1.62ubuntu1.1" by rebuilding the source code on a PPA along with d-i " 20101020ubuntu318.40" in order to get a mini.iso.
I confirmed the mini.iso override the hostname as expected if "netcfg/hostname" is set and that the override occurred before the disk configuration for the vg name to match with the overridden hostname, if LVM is use.
I have tested on Trusty with "preseed - 1.62ubuntu1.1" by rebuilding the source code on a PPA along with d-i " 20101020ubuntu3 18.40" in order to get a mini.iso.
I confirmed the mini.iso override the hostname as expected if "netcfg/hostname" is set and that the override occurred before the disk configuration for the vg name to match with the overridden hostname, if LVM is use.
Eric