My advice: move away from native mouse_maps's. Assign a dummy bmap to mouse_map (1x1 pixel black or tga with alpha = 0) and use a panel that is being synchronized with the mousepointer.
By this you are able to circumvent the size limitation and any other sideeffects, plus you are free to use other cool features you can do normally just with panels, for example scaling. When you implement a resolution-free display with scaling all panels and stuff, the panel solution for mouse cursors will be useful, because you can scale the cursor panel as well so that the cursor is itself resolution independent as well.

If you want to keep the default button behaviour for panels, the attached panel needs an offset of +1/+1 pixel, other wise it would cover the mousemap/cursor and the buttons/entities can't receive mouse events. If you are smart enough, you can write your own mouse event functions and callbacks so that you have full functionality even with a panel solution. Just in case you dont care about offsets (maybe the cursor is under the attached panel).

I had also a lot of trouble with hardware cursor synchronization, so, I abandoned this and made it on my own which is quiet more satisfying than the GS mouse support.

Cheers
Christian