Depending on what the MMU would allow (I know so very little about low level ARM) you could probably map an area of ram to the same address as the old screen memory and make a compatibility layer. This still assumes that ARM-1 is binary compatible otherwise. So you might be able to get some kind of help from the hardware for at least video, but I have no idea about sound and input. I believe that this kind of remapping should be doable in linux so I'd assume that it could be made into a layer in RISC OS also.