It's pretty simple and will let you use a custom name for your stuff.
What you need:
GXT editor http://www.gtanetwork.it/zone/pafiledb.php?action=file&id=3833 IMG Tool 2.0 http://grandtheftauto.filefront.com/file/IMG_Tool;44920 TXD Workshop 3.5 http://www.gtatools.com/
Open "player.img" and find "tshirtwhite.txd". Keep it open. Extract it to a new folder. Open it and extract the texture as TGA into the same folder. Edit the texture to your liking using photoshop or a similar program. Import the texture into the TXD and save it. Go back to "player.img". "tshirtwhite.txd" should still be selected, click "command>replace", find the TXD you just saved and double-click it. Close IMG Tool. That takes care of the texture.
Open GXT editor. It's a 3 pane window, 2 panes are on the left, one on top of the other, and the other one is a big pane on the right. Open "american.gxt". You can find it in "C:\Program Files\Rockstar Games\GTA San Andreas\text" or wherever you installed the game. Click "search>find value" and look for "white t-shirt" (without the quotes). You should see it selected in the lower left. Edit it on the right. Name it whatever you want. Click something on the bottom left to update the list and then save it and close the editor. That covers the text.
Start the game, go to Binco and your new shirt will be available with its custom name.