AHAHAHAHHAAHHAAA! I’ve wanted to do this for over ten years! Finally!
Look upon my works, ye mighty, and despair!
(Added three float arrays as custom properties, using them to drive the mapping in the shader via attribute nodes. Can now easily detatch faces as their own objects and change their texture mapping with custom props, then edit them with nearby objects with Blenders newish multi-object editing)
I’m very happy.
Credit to @roxya on the OpenVFX discord for pointing me towards custom properties.