Has anybody ever converted a [DSM] to rear-wheel drive (RWD)?

Yes. Dave Buschur had what was perhaps the first-ever RWD DSM back in 1998. To do it, he rotated the engine ninety degrees in the engine bay and adapted it to a Powerglide racing transmission. For more details see Dave Buschur's biography.

While it is possible to do this type of conversion, the difficulty level is (to put it mildly) very high.