Your posts seems a little confused.
- No, no distro includes ROX. I've been discussing with Peter "Mr Flibble" Howkins building a distro based around ROX but I confess I've not done much with the idea as of yet.
- It's not really viable to boot a PC from an ISO stored on hard disk. However, if you want to play with ISO images, most PC VM programs can do it. MS' VirtualPC or VMware Server are free, run on Windows, and can boot an OS from ISO inside a window. On Linux there's VMware Server, QEMU, KVM, Xen and others.
- I don't see what possible relevance there is between UPP (porting Unix apps to RISC OS) and ROX (a RISC OS-like desktop for Unix). What were you thinking of?
- Cross-platform apps might be doable, but we are talking about 2 totally different OSs with only a cosmetic resemblance. It would be decidedly non-trivial. Why would you want this? It might be more relevant to ask if a RISC OS emulator for Linux could be made capable of running apps inside a RISC OS window on the Linux desktop, in the style of Mac OS X's Classic support, OS X VMware's Fusion or Parallels Desktop's Convergence.