For starters, the Saturn 128K card was a RAM expansion card produced during the Apple ][+ era. From what I recall, unlike the later AUX slot RAM expansions for the Apple //e, it did not provide bank switching of the full 6502 address space, but rather multiple pages that were mapped to a block of memory on the ][+ (LC space?), more like the slinky RAM card that Apple produced. That being said, the question isn't really about its mechanics and I just put this here for some background.
The question really is if you ran a version of ProDOS that supported the Apple II/II+ machines (i.e. v1.0.x), would a Saturn card be recognized and used as a /RAM device (a.k.a. slot 3 drive 2)? Or was that a feature reserved for the extended 80-column card of the Apple //e (and naturally the //c and IIgs)? Would it make a difference if the card was installed in a //e?
If the card wasn't supported in the original ProDOS versions from Apple, did this get picked up by the recent releases of v2.4.x?