Yesterday I decided I would try out chainloading grub2 from linode’s pv-grub after reading https://notes.pault.ag/linode-pv-grub-chainning/.
Even though I had successfully built the shim using the instructions, upon booting with a generated config I ran into issues with grub2 not knowing how to deal with my LUKS+LVM disk images.
After more searching, I came across a guide on http://blog.cyplo.net/2013/11/10/booting-gentoo-lukslvm2systemd/ which dealt with setting up grub2 to work with an identical root setup to my own.
The relevant part of the guide was the grub config,
nimbus ~ # cat /etc/default/grub GRUB_DISTRIBUTOR="Gentoo" GRUB_PRELOAD_MODULES=lvm GRUB_CRYPTODISK_ENABLE=y GRUB_DEVICE=/dev/ram0 GRUB_CMDLINE_LINUX="real_root=/dev/mapper/Nimbus-root crypt_root=/dev/xvdb dolvm rootfstype=btrfs rootflags=compress=lzo" GRUB_DISABLE_LINUX_UUID=true GRUB_DISABLE_RECOVERY=true
After setting the config up and generating a new config file, i was thrown into the familiar passphrase prompt on the next boot without any errors! (Huzzah!)