Riscose doesn't seem to have been touched in about five years. However, the most recent update does sound very much like it could be useful. (As could Matthew Bloch's web hosting service, bytemark).
The difference is that the major part of their work (as far as I can tell from a quick look at their site) was on the ARM emulation and, apparently, BASIC, too. These have been superseded by QEMU and Brandy, and the SWIs I'm most interested in are more WIMP oriented than command line.
There's also the open source shared C library by Graham Shaw, which will probably will come in useful.
Do you mean RISC OS (ARM code) Relocatable Modules, or a ROLF native system? For the former, it's probably possible using shared memory and some synchronisation between instances of QEMU; for the latter, I think shared libraries are probably the way to go in most cases.
Bear in mind that, sitting on top of the Linux kernel, applications can use pre-emptive threading and non-blocking I/O, so many of the uses for RMs are not needed under ROLF.