Mathieulh has hacked the latest fw I believe, so there's a very high chance the same kernel exploit, or an unreleased one will work up to current latest fw
Afaik unless a game relies on a specific function call or feature on newer firmware then a modified .elf or whatever it is called should allow the game to run fine, just a case of editing the required firmware to read from X to Y. Obviously if a feature the game requires isn't available on the earlier firmware then the game should just crash.
The very old cfw is possibly 4.05 with a modified pup or elf file, either that or running on a ps3 maybe?