BASIC and TWIN were written originally to run on an emulated ARM, long before any silicon existed. So I think druck's correct - you don't need many SWIs to get them running.
(Assemble-time) A500 support was finally removed in RO3.5 or 3.7, I forget. I suspect 'Brazil' support was removed when it evolved into Arthur.
On a slightly different topic, I had a look at my Springboard manual (ARM dev. system on a PC ISA card). It lists the SWIs it supports, and points you to the BBC Master's Technical User Guide for more info. Thought that was slightly amusing.