As you can see, the plan has a hole, marked with ???. The best I've been able to come up with so far is running a VNC client on the mac and requiring users to run a VNC server, then using ssh+zeroconf to let people start up a VNC session via the command line...? That sounds terrible -- there's gotta be something better. It sorta sounds like the sort of thing X was made for, but (a) what we really want is to view a portal of someone's screen, not have them push a program over the wire; (b) I haven't quite thought through how mouse/keyboard would work; and most importantly (c) we have lots of different platforms, most of which don't really speak X.
It'd almost seem like not that hard of a program to write something that lets you rectangle-select an area of the screen and push that bit over the network. (For example, AFAIK this is more or less what the XDamage extension exists for.) But maybe this has been implemented before? O lazyweb, tell me it has.