OK, I managed to get the codes used in the AnyRegion Changer 1.1b mod05 by QLeap which added 'ENHANCED' Region Free, meaning it added 'Disc Area Check' to its patching - for modchips that force-patch for a region (like D2SUN, where you can't have the chip be on 'open', thus always applying 'RegionFrii' type patching), the GC Region Free with Video Mode patching doesn't work.
My Metroid Prime USA would show a black screen instead of showing the video in the intro, on my PAL Wii, unless I flashed my chip to have an 'open' region and not patch for one, in which case the GameCube Region Free with Video Mode patching would work and not clash with the chip, and would show video.
For testing purposes, I flashed my chip back to a firmware which forces the chip to patch for PAL, showing my Metroid Prime USA without videos in the beginning. With the Disc Area Check codes I got from the AnyRegion Changer 1.1b mod05 it shows video now, although I have to admit that while testing with PreLoader, disabling and re-enabling the settings, sometimes it was a bit iffy and worked/didn't work properly, but with it on now it works just fine for me.
These are the codes:
[Disc Area Check]
version=290
offset=81377B8C
value=0x4800001C
[Disc Area Check]
version=290
offset=81377BB0
value=0x60000000
These are for PAL 3.2E 290 System Menu..