Actually, feeding old monitors the wrong frequency for an extended period of time can damage both the horizontal output transistor and the flyback. The worst case is actually if you see a distorted picture instead of "nothing"; that means the monitor "sees something" and is pushing itself way out of whack in a desperate attempt to sync with it.
If the monitor makes an audible squealing noise then you're *really* asking for it. I've never actually managed to kill a monitor by overdriving it but I have heard some older multisyncs make some really pathetic/scary noises, so it's not like I haven't tried.
So, short answer: No, don't bother even trying your IIgs monitor on a Mac.