It was never officially supported by Sun (the SM100 modules were only supported in the 600MP, including pair of them, which is a nice vintage configuration), but it does work in practice - both the host and the module are sufficiently MBus Level 2 compliant to work nicely together, and for some reason (probably "didn't remove it from the know-working code in the 600MP") the PROM supports the modules...
I tried NetBSD 9.0 on the config just to see if the software would actually work as well... and it did :-) (probably also for compatibility with the 600MP?). Performance-wise, it doesn't make any sense though, in particular as I luckily do own a pair of SM512 should I need a quad-CPU SS20. But those run way too hot for my peace of mind... (and work fine with NetBSD as well!).
Cordially,