the geo has a lot of tire clearance so i'd suggest an auxillary cable to lock the wheels with and then you'll be free to use the U on the frame, or try it on the seattube and rear rim if the post is thin enough. as long as the lock can fit around one frame tube and the locking post you should be okay. a mini-u is a huge weight difference, and for me at least is worth the extra weight, even with an auxillary cable.
like this:
