Yeah, we could then have a subforum for RTOS embedded systems like the Atmel AVR which do not have MMUs.
You'd be much better served for that sort of thing by using an existing dedicated forum like the one at http://www.avrfreaks.net/
I don't currently see any good reason to add sub-forums for different architectures. If a question comes up about embedded development, then you'll probably get directed towards an alternative resource if it's something that doesn't fit in here. As XenOS suggests, there isn't enough traffic related to architectures other than x86(-64) to justify the split.
As for the OS Development forum: As long as the questions are sensible and the poster has followed the general forum rules (research before posting), questions are welcome on 16/32/64 bit and flat / segmented memory models. There is, of course, the proviso that if the question is about an unusual / outdated mode or memory model then: a) fewer people will be able to provide sensible answers and b) the OP will probably be advised against using said mode/model unless the reasons for doing so seem well thought out.