Hmmm. After an evening's worth of poking around I don't think GB will work for this very easily. There are some odd-meter options (5/4, 7/8, 9/8) but nothing in 11, nor any means I can find to define or create a measure that isn't one of the predefined choices. So the work-around would have to be like the old drum machines: program enough bars in 4/4 or 3/4 until arithmetically it comes around to line up again. Easy for 10 BpM, just make 2 bars of 5. Not so easy for 11BpM. Best choice is something like 11 bars of 3/4 (33 beats) to make 3 bars of 11/4. If you have a 2 measure pattern in 11, you need 22 bars of 3 BpM to make the loop. And god help you if you change meter within a piece of music.
So Garageband is not the tool for this, but other things exist and I have my beat box and dub buddies thinking about what would work best within common software that doesn't require investing $$$ in something like LogicPro. When I have a good solution, I will post.