They make a PS2 to Gamecube adapter for the Wii. It plugs into the GC port, I use it to brawl. But it also has 2 other adapters so its 3-1 all for using PS2 on 1.X-box 2.Wii/Gamecube 3.Computer. I paid 18.00 for it. Not bad.
Mayflash is going to create a 2 port Wii Classic Controller to PC USB adapter sometime around the end of September or October. It will be a straight up USB adapter like their PSX to USB models, so it won't require all that extra stuff like the Classic Controller + Wii-Mote + Bluetooth + Glovepie set up. It'll be as simple as just plugging in a Classic Controller and setting the joypad inputs in the Emulator (or Joy2Key if it requires keypresses). For those interested in such an adapter, keep checking Mayflash's site.