Well, assuming you can get (or have got) Hijack on there, Mark's made a Unix-y tool to do upgrades without needing a serial cable, and since MacOSX is awfully Unix-y, it ought to work. This is good since it seems finding a usable USB serial cable for Mac-to-empeg connection is kinda difficult.

I'll let someone else fill in the details, but I think that this is the way to go.