On Debian-based systems including Ubuntu, they offer an initrd based Dropbear (lightweight SSH server) which can be used to connect and authenticate. Involved a bit of custom scripting last I checked though, but a possible solution if that's a requirement for you.

For such systems I consider the primary root file-system to be part of the 'bootloader'. Everything that I want to keep actually secure is inside of the encrypted PVs backing LVM volumes.

Yes, this presents a security risk in that someone could (offhand I think the term is 'evil maid'?) attack the root filesystem, but they could still have done that to the bootloader anyway.

Remote interaction is then required to bring the VMs on that system up.

They could not have done that to the bootloader because of SecureBoot.

With secureboot on Linux you can secure as much or as little as you want. On my system, grub isn't even safe, only the shim that load grub is secure. But I could set it up so the kernel is secure, have the kernel only load verified initrd, and then have the initrd check the root filesystem.

I don't, but secureboot can detect changes to the root filesystem if you want it to. I think this generally requires setting the rootfs to mount readonly.

You could put a daemon on the initramfs (say, ssh) that allows you to remotely provide a key/password for decrypting the disk.

This would certainly be more work to set up than vanilla full-disk encryption, though.