Add support for partition table inside LUKS on boot

I'd like to have GPT partition table inside LUKS encrypted disk (not partition).

It looks like this:
vda 252:0 0 100G 0 disk
└─test 253:0 0 100G 0 crypt
  ├─test1 253:1 0 20G 0 part
  └─test2 253:2 0 80G 0 part

However, I'm having difficulties achieving this on Ubuntu because of its boot scripts.

2 steps that I needed to make this work.

First is to remove file system type check, since partition table is not a filesystem, so I've edited /usr/share/initramfs-tools/scripts/local-top/cryptroot and commented-out a few lines like this (snippet from 18.04, 18.10 is a bit different but not much):

# if [ -z "$FSTYPE" ]; then
# message "cryptsetup ($crypttarget): unknown fstype, bad password or options?"
# udev_settle
# $cryptremove
# continue
# fi

Second step was to add partprobe to recognize partitions from LUKS container, so that I can mount them in /etc/fstab.
For this I've created a hook:

root@fs-experiments:~# cat /etc/initramfs-tools/hooks/partprobe
#!/bin/sh

cp /sbin/partprobe $DESTDIR/sbin
cp -L /lib/x86_64-linux-gnu/libparted.so.2 $DESTDIR/lib/x86_64-linux-gnu

And a script:

root@fs-experiments:~# cat /etc/initramfs-tools/scripts/local-premount/partprobe
#!/bin/sh

case $1 in
prereqs)
 exit 0
 ;;
esac

partprobe

I'd like to see partitions being supported out-of-the-box so that there is no need to edit system files for such use case.

P.S. I know about LVM and the fact that is it supported out-of-the-box, but I found it extremely painful to use for this use case unfortunately.