I didn't know the Xbox had a HD15 port, that's pretty good. Although of course Microsoft does not permit running anything else on the Xbox other than it's built in OS. That's not to say it's impossible of course, but if you're deciding to port to another processor, you can choose one which permits every conceivable hardware permutation (x86), or you can go with one which only permits running on non-upgradeable hardware, no laptops etc (PPC consoles). If we were talking about the Amiga, which is already PowerPC native, then the PS3 especially is a no-brainer, particularly as Sony is perfectly happy for you to install another OS on it (and even tells you how).
But as we're talking about RISC OS, a hypothetical re-write to another processor may as well be done in such a way to allow many different CPUs, if you're doing all that work anyway, may as well do it in a cross-platform way with a runtime (like Inferno, Java, Intent etc.) or in a more traditional compiled-for-each-arch way like BSD, OS X, Solaris, Linux etc. That way you can just pick whatever CPU suits you best, regardless of it's architecture.