jess: 26bit and 32bit sources aren't different instruction sets, everything in C will be exactly the same, and of the assembler its 99% the same ARM instructions and 1% conditional macros in places where a couple of different instructions have to be used on 26bit machines to maintain the expected flag preserving behavior. As said it doesn't help one iota in making it portable to other architectures.
If you feed the ARM assembler in to a cross compiler you would get the same result as the output of an emulator JIT, native instructions but something that still needs an emulation of specific legacy hardware. So you either still need to provide this, or more likely recode the hardware specific parts. As mentioned in RISC OS 5 most of the low level processor and main bus bit twiddling (including timers) is in the HAL, which would be recoded for the native architecture. The higher level hardware specific module would need adapting for the specific southbridge and video card, or more likely the unified model provided by a virtualisation layer.
Even with suitable tools there is no free lunch, it would be a lot of work, and even after you've done all that, you still need to provide an ARM JIT emulator to run all the legacy RISC OS applications which will never be converted for the new architecture.