Reality Factory

Can I make a player change shirts In-Game?

Yes, in game you can have the players actor perform a texture swap, which will swap out its original look for another texture with the desired look.

or if you feel brave (not sure if this is technicaly possible with animations) use the entire body as weapon slots, they wont fire bullets or anything it just means you can attatch a model to the body, so say your person is a small block and then attatch the legs/arms/body/head ect. to the block so it covers it up then you can change them like you can with weapons.

Thats an idea, but inspired this idea:
You could create the shirt as an object and have it just as an object that spawns into the game upon command. This is possible I believe with the newest RF release, or maybe you have to add code to the source yourself, but it is indeed possible now to add entities into the game world while the game is being played.

If that is too much you could have the shirt be a pawn that toggles visibility and spawns and attaches at the right place on the player.