The ZeroLink docs are still here.
It doesn't work with MacMAME in any version of Xcode that I've tried.
The Xcode "Upgrade to Native Target" converts old PB jam-based targets to the new system that supports ZeroLink/Fix&Continue. It doesn't have anything to do with Carbon vs Cocoa. MacMAME is already using a native target.
First thing I'd try is building the Profile build (basically the same as the Deployment build, except with debug symbols) and run that. If it quits, look in Console and ~/Library/Logs/CrashReporter/MacMAME.crash.log to see if there's any explanation.
v-sync works fine in 0.77u2c and 0.84 for me. The other video improvements (as described in the xcode note) are: fixed dark border around GL window, correct aspect scaling of multimonitor games in fullscreen mode, and a phosphor persistence slider.
The other important improvement in the Xcode build over the official 0.77u2a is that it yields time to other apps correctly, so Ms Pac Man takes 10% of the CPU instead of 100%, and your fan(s) don't come on.
The big tradeoff is that it uses the C cores, so if your Mac was too slow, or barely fast enough to run a game fullspeed before, it will be slower now.
For the newer games, Tekken (System 11) runs at a pretty solid 30fps on my 1.25GHz laptop, so a new G5 ought to be full speed. This is without sound, though. On the other hand, Virtua Fighter Kids (Sega Titan) runs at about 8 fps.