I'm beginning to boggle at you. fork() requires advanced memory management and multitasking abilities. RISC OS doesn't have these. Adding them would be as much work as reworking the dozens of places RISC OS keeps state.
I can't think of a compiler anybody actually uses on ARM that doesn't compile switch statements to jump tables where possible.
I can't think of an OS anybody actually uses that has fork that doesn't do it copy-on-write. (And then you go and tell me what copy-on-write is. Boggle.)