You think it would be as easy as running the iLife ’05 installer and doing a custom install for iMovie. But no, the iLife installer grays out the iMovie option. The solution to this is to remove the “receipts” from iLife ’05 and iMovie. Once you remove the receipts then you can install iMovie 5.
You can find the receipts in the
[computer name]/Library/Receipts/ folder.