Would it be feasible to occasionally measure the actual render time required per avatar and block / reduce avatars that exceed a threshold (GPU, CPU time, VRAM usage)?

The advantage is that it would give accurate measurements for any given hardware, whereas the current measurements are just a rough guess. This way "good" avatars can be blocked if they have an expensive shader, while still allowing "poor" avatars that have slightly too many polys or materials (from e.g. clothing options, not all active at once)

Those measurements won't have to run every frame, for every avatar. It could be done when the framerate drops (either apruptly, or below a threshold) (ignoring lag spikes, such as avatar changes, or taking high res pictures)

Depending on the details of the performance measurement it could not just block the avatar entirely, but just disable custom shaders, physbones, downscale Textures.