[steven]

I think RickOShay is on to something-- it depends on the game. I think that's true for a few reasons. For starters, if the game is really old, then kanji won't typically be used. I suspect this is due to low memory just as much as it is due to the fact that pixel counts can be low thus making kanji impractical. Speaking of memory, I've noticed that a lot of Famicom era games have either just katakana or just hiragana (with some of the alphabet mixed in). I've seen kanji on gameboy/gameboy color games and it is frankly just really hard to read. A native speaker would have an easier time because they would know what to expect-- so even if the kanji would be practically illegible out of context, within the context of the game it can be read.

Within a single game, you will also likely encounter many styles of writing. This is probably to indicate a personality of a character. I've seen games where certain characters will use special endings that are not in the Japanese language to begin with. I've also seen games where certain words are 'spelled' with katakana while others are 'spelled' with hiragana or even kanji.

I mean, I could go on about this... but to make it short- all rules aren't out the window, but they're certainly up in the air considering a character or situation. Speaking of rules, chryuop pointed out that there are uses of katakana beyond what you stated in your question... and I think those rules are invaluable for your Japanese (gaming or non-gaming related). One last note- I've noticed that a lot of games will include a lot of "spoken" Japanese grammar. By that I mean you will encounter a ton of stuff that is usually not written (particularly in any dialogue sequences).

Is there any/are there any game(s) you have in mind?