Ok, now I understand what you're trying. There are a few options as I see it:
1) if you only want to use iMovie, you have a few options:
A) you'll probably have to record the event, then change the timing on the recording multiple times to get it to the right length
b) record the entire event, then go through it knowing that you need one frame out of every 3000 frames. so moving ahead in the footage ~3000 frames from the previous selected frame and choose your next frame.
2) If you're willing to use other software (I can't guarantee if either of these titles will work on your ibook) check out:
a)
Boinx Software - iStopMotion 2 - this program will allow you to setup your computer and camera (and according to its list of supported cameras, it supports the hv20) and have it capture a single frame every X seconds/minutes/hours/etc.
b)
FrameByFrame - I don't know if it allows preset recording times capturing a frame every so many seconds or if it specifically requires you to be there to trigger the next frame (it's original design was for stop motion animation, but from what I had read elsewhere it can be used for what you're trying to do)
3) See if that camera (I don't know about the hv20, some camcorders have this feature some don't) has a timelapse function that can record 1 frame every x minutes, or get a cheap used camera that can so you can interleave your footage from the timelapse camera with your main camera (may require some color work to get the tones correct).
Those are the options I can think of off hand - maybe one of those might help you out?