Have any of you tried this technique of using hydrogen peroxide plus OxiClean to restore discolored diffusers?

ahh the old retrobright project.
the few badly yellowed diffusers i would have tried that on were also brittle and falling apart.
plastic was totally killed by heat and uv.
i have used it on thicker stuff with good results.
i might try it on some old jensen rv fixtures with yellowed plastics before i replace the lenses.

The "RetroBright" and similar treat just the surface and just removes the brown bromine released from the flame retardant additives, otherwise it just bleaches the surface. That mean once the yelowing is from an other origin than the bromine "prespiration", mainly when it is not limited to the surface, it won't help. Moreover it can oxidize the surface, which mean it won't be clear anymore.

As the bromides are not used in the clear optical plastic in the first place, I don't think such bleaching agents will work on the materials used for the lenses. The discoloration usually come from the material degrading in it's depth and there no chemical could reach.