Teksanity has published the changes necessary to the code of iMove 7.1 in oder to allow it to run on G4s. On the bottom right of the page there is also a discreet link to download a patched version.
For your information, the patch consists of changing the hex code "7C 08 02 A6" with "4E 80 00 20." In simpler terms, it modifies a branch in the code. To make a text analogy, it would change the phrase "if the processor is a G4 or a G3 go to address X" (and that address would give an error message) to "if the processor is a G4 or G3 continue the execution."
It is a classic technique that we used to modify the first iDVD which only launched on machines with Superdrives or with DVD Player so that it would play from an external DVD drive.
[translation by jwa]