Update, for those who may be interested:
More in hope than expectation, I tried hooking the G2 up to the MacBook and running ImageCapture... after a few false starts (mainly due to finger trouble), it worked
(The 40D didn't like it- it went into a "busy" loop and stayed there until I switched it off
It's not ideal because of the way the G2 works- it has to be in "playback" mode, and then you've got no control over the camera settings (zoom level, flash on/off etc). But as a trial I pointed it out the window at the cloudscape (fortunately it's fairly windy here today) and let it take a picture every 15 seconds while I watched the second practice for the Spanish Grand Prix
Then install the QuickTime 7 player from the Snow Leopard CD, upgrade to Pro and follow the instructions as for the GoPro (thanks metalhead).
So now I have a nice little 24-second time lapse video of the clouds hurtling over me for 90 minutes- happy bunny
In a few days when I've got more time I'll check out the trial version of iStopMotion (thanks Ian) and see if that's better for camera control.