I did this with Junxion, a program that allows you to transform USB and wireless game controller input into MIDI data. I’ve seen demos of other MIDI rock band instruments online, but with the Wii instruments already in USB form this has to be the easiest setup yet. This also seems to have the lowest latency. Junxion is not free but a demo can be downloaded here:
I’ve also used Junxion to turn other game controllers such as DDR dance pads into instruments. I’m using the old PowerPC version 1.4, but version 3 should work fine. Junxion 3 also now works with Wii remotes.
I just used the Studio Tight Kit preset in Ultrabeat in Logic for the drum sounds. Junxion automatically makes an input port which Logic recognizes and listens to.