Here's a photo of an XY stage made in Lego:
It's much bigger than I'd wanted, largely due to the weight of the Keling stepper motors I'm using. The lego beams can't support the motors unless they're assembled into a box-structure around the motor mounting face.
I've used Lego railway tracks and wheels to allow the axes to roll. The white parts are mounts for the opto-endstops (they simply bolt on). Since the gap in the endstop sensors is too small for lego, I will simply bolt short pieces of L-section aluminium to the lego such that it slides through the gap and triggers the sensor.