In principle, I don't see why not. In practice, yeah I can see it. In both cases, can't know without checking, and I haven't. Bevy is pretty cool though!

My drive-by suggestions would be more along the lines of https://github.com/mattwparas/steel, or the very ad-hoc embedded language of https://github.com/elkowar/eww, or perhaps most pertinently https://kdl.dev/

Maybe one of those would be minimal enough for the rest of the engine to not manage to get your way, as frameworks are known to do. As always, happy hacking!