You can load up ocarina codes using the USB loader, right? I don't use it, but if you can, then this should work -
Download this, put it in your 'codes' folder in your SD card. It's the Screenshot Loader/File Replacement .gct file, it's what enables the screenshot hack to work
http://www.box.net/shared/yrfhbzhxii
Then, find a cool program called 'brawlplustweaker', and download that. Once you've got it on your computer, you need to track down SDKEY and SDIV (which I can't share, but
google should help... make sure you use the right keys, or it won't encrypt properly) and place just the keys (which are each one-line strings of what appear to be random letters and numbers) in files named SDKEY.txt and SDIV.txt, and put em inside the same folder you've got BrawlPlusTweaker in.
Download the .gct of whatever Brawl+ build you're using (
here's the latest stable build), then use BrawlPlusTweaker (the gct to bin button at the bottom) to convert that file into a screenshot, which you then put in the SD card "private/wii/app/rsbe/al" folder, under the name al_(anything can go here).bin
Enable ocarina codes when booting the game and navigate to the screenshot in-game and everything should be cool. Go to the 'Brawl' menu to check - there should be more character slots.
This method also works with DVD backup loaders, very useful if you no longer have an actual copy of Brawl like me. I thank the cool friendly folk at Smashboards for this method.