No, there isn't any USB gamepad support in libogc. It might be possible though, just like USB mouse and keyboard are actually supported, all you need is a proper driver (and someone willing to code it). And honestly, it's not so "surprising", the Wii already supports various controllers natively (incl. retro adapters for SNES, NES, GEN, etc), gamepad support through USB was clearly not a priority. It would still be nice to have it as an alternative though.