I have not. But I have pondered the same question.
FYI - NOS GM versions of the rubber rear main (and NOS crosses to other manufacturers) are NOT hard to find on eBay. IIRC, there’s an OMC version of the same seal that is pretty widely available.
Generally speaking, when it comes to rubber parts, I look for NOS whenever possible. Old rubber formulations seem to be borderline eternal. New rubber starts going bad before it’s even removed from its box.
That approach has served me pretty well with things like suspension bushings, motor mounts, etc. But I’m not sure how transferable it would be to something like a rear main.
