I'm sure I deserve plenty of shame, but MAME OS X most definitely has pause and screen shots. Didn't know core MAME did frame advance. If it's one of the standard keys, it probably works. Try it.
A couple of general comments. Complaining that MAME OS X doesn't do this or that on the *MacMAME* forum is hardly a good way to let the right people know about the problem. MAME OS X is far from perfect, but it pretty much does everything *I* want MAME to do. This means I don't test every edge case. So if you find something wrong, by all means report it. Even if I don't get to it right way, I'll at least put it on the todo list. Andy maybe even someone else will get to it before I do.
Second, resurrecting MacMAME is going to be a *lot* of work. The core has changed so much that it's far from a trivial task. In fact, I'd say it's just not worth it. That work would, in my opinion, be better directed at improving MAME OS X. I did look at reviving MacMAME before starting out on my own, but I felt using newer OS X technologies would be worth the effort of re-writing. And last week at WWDC, I was somewhat vindicated in this decision. In case anyone missed it, 64-bit Carbon is dead. It's not happening. The way forward on OS X is Cocoa. 64-bit should produce a nice speed improvement, when we can take advantage of it.