Apologies if this has already been considered @Jip but can the original texture be replaced with a higher resolution one that makes room in UV space for new HQ geometry? For example, take a 512 texture and place it top left in a 1024 canvas, divide base model UV coordinates by 2, allocate new UVs in the 0.5 - 1 range that is now available for use in the 1024 canvas. Of course that still leaves the problem of actually authoring these additional textures and my instinct is that it would involve reusing repeated patterns from the original. The problem of albedo being mixed in with the baked GI could be solved in principle with a high/low pass filter as a starting point, making a plausible albedo texture without GI contribution available for copy-paste over the new geometry, and blender can be used to provide the GI for these new elements.
Basically I'm asking if this sort of solution is acceptable from your developer perspective, fully aware that it creates work on the art side but ready to contribute. I know it doesn't address animations but could be part of the answer if you decide to iterate on HQs further.
