I know. Here's the code for Path of Radiance (US).
Make Game Save Copyable (donny2112)
Note: Start up your game and make a save occur (e.g. Suspend, Save). You can now copy the save file to another
memory card on the GameCube memory card manager screen. You only have to use this code once to make the
save copyable from now on.
I wonder if anyone ever made such a code for USA Gotcha Force...
Anyway, back on-topic, this code unlocks all the trial map characters:
Unlock All Bonus Characters (Trial Maps) (donny2112)
As for all Support conversations, there's a Radiant Dawn save file over on GameFAQs which has them all already unlocked.
Note: I haven't tested either code or the above save file personally.