Broadly speaking, let me divide this into two kinds of questions: (a) conceptual questions about some particular architecture (e.g., Mano, MIX, etc.), or (b) coding questions, involving writing code for that architecture.
I would propose that coding questions should be off-topic, even if they are asking for code targeted at a theoretical model. For instance, "how do I write assembly code for memcpy(), in the Mano assembly language?" would be off-topic, as that's a coding question that calls for us to write code targeted at Mano assembly. Same for MIX, or MIPS, or x86, or any other assembly language, regardless of whether the underlying platform is real or theoretical.
Why? I don't see much difference between "write me code to do X, in MIPS assembly" vs "write me code to do X, in Mano assembly" vs "write me code to do X, in MIX". The first of those three is already established as off-topic. I suggest that the other two are essentially equivalent and should also be off-topic. Askers could be directed to Stack Overflow for those sorts of questions.
I have no position on whether conceptual questions about a theoretical architecture/platform should be on-topic or off-topic.