if i am understanding you correctly, yes. if you use brawl from a usb, i take it that means you have homebrew installed with either cfg usb loader or usb loader gx? you should be able to do this pretty easily, it'll just require renaming some folders but you can install a homebrew app for that so that you don't need to put the sd card back into the computer to switch every time. basically this is how your SD card might look:
apps
MYcodes
MYprivate
PMcodes
PMprivate
background.png
boot.elf
gameconfig.txt
so you take the PM codes and private folder and rename them to PMcodes and PMprivate, as you can see, doing a similar thing for your own mod. then using something like WiiXplorer launched from the homebrew channel (located
here), every time you want to switch, go into WiiExplorer, (let's say you want to play PM) and rename PMcodes -> codes, and PMprivate -> private. then launch your usb loader and make sure ocarina cheats are enabled, and it will load PM. then if you want to play your own mod, restart the wii, go to WiiXplorer, rename codes -> PMcodes, and private -> PMprivate, and then MYcodes -> codes and MYprivate -> private.
the downside to this is that you're loading totally separate private folders for each mod, so this means that you can't use the same music files or character textures (for example) between each mod without having duplicates in both of the two private folders
