If RISC OS 5 is 'the better OS', as druck suggests, they why did STD pay RISC OS Ltd to convert Select to the A9, rather than taking the already 32bit RISC OS 5. If the HAL is any good, that should have been a much simpler (and cheaper) task?
I know nothing about the Castle HAL, but i will point out that RISC OS 5 currently only runs on one platform. The Castle Iyonix.