Yes, it's absolutely possible to recreate the environment for testing this without MAAS -- there's nothing all that special to it, chainloading *any* image should work and maintain a Secure Boot-verified chain provided all the links in the chain validate images.
This looks to be pretty clearly a bug in chainloader's validation of images, it used to work, but only because it wasn't actually verifying much of it in the first place.
Yes, it's absolutely possible to recreate the environment for testing this without MAAS -- there's nothing all that special to it, chainloading *any* image should work and maintain a Secure Boot-verified chain provided all the links in the chain validate images.
This looks to be pretty clearly a bug in chainloader's validation of images, it used to work, but only because it wasn't actually verifying much of it in the first place.