A new thought... virtualisation is quite good nowadays: you can run one OS inside another with relatively little hassle or performance hit. I've thinking of things like VMware, QEmu, Xen etc. How about if VirtualAcorn came up with a basic Linux distribution that ran VRPCforLinux? They would ship a CD with a disc image of this on. Also shipped would be emulators/virtualisers for a number of target platforms (Linux/x86, Windows, Mac OS X/x86). So you could either install the basic Linux distribution resulting in a standalone VRPC machine, or install the virtualiser and emulate Linux emulating RISC OS. That sounds slow, but modern virtualisers have little speed impact if they're on the same CPU architecture, and forthcoming processors will have specific support for virtualisation. Any changes to the underlying OS/libraries/etc just require an updated virtualiser which might even be part of the OS distribution (eg QEmu comes as a Debian package).
The problems I can forsee are graphics card and host access, but those aren't insurmountable.