The way I did it was to create tiles that are larger than the required size. Then use the marquee tool (which does snap to guides) to crop them down to the correct size. 100px in my case.

Note that if you have tiles that are an odd number of squares on one side and an even number on the other then they won't rotate properly within maptool. You'll need to save two copies, one for each orientation.

Looking forward to seeing what you come up with!