OP: it would help greatly if we knew what your use case actually was, because your demands are kind of strange.
I'm starting a new coding project, based on an inhouse GUI lib that has two backends: one for X11, and another for wxWidgets/Cocoa and wxWidgets/Win32 (yes, wxWidgets has a X11 port too, but our X11 backend is very lightweight -just plain C, developed in the golden times of IRIX, and maintained until today).
I'm interested in keeping this new application optimized for slow hardware, and given that its only requisite is X11, I thought compiling for an old UNIX in parallel would be a cool idea. I didn't mention it, but I also prefer a 32bit machine as a minimum, because 8bit and 16bit machines tend to have complicated memory extensions mechanisms -near/far pointers, bank switching and related magic- which greatly complicate development.
Yes, I understand is a somewhat uncommon search (although not too rare, because in the process I've found quite a lot of people interested in old Unices).
There's something I find very strange, and that's why on Earth would anybody develop an emulator that doesn't emulate speed (at least in an approximate way). I think the main point of an emulator is recreating the original user experience with the machine, and that can't be done if you don't try to provide the original performance (yes, I know it's sometimes useful to run old software as fast as possible, but if you emulate speed it's usually easy to provide configuration settings for increasing the original speed of the machine).
So, I feel closer to those emulators that try to keep close to the original speed, such as MESS, for example.