shift tower werk again

I know we were talking about working on shift tower/rail mods to stop the common leaking problem from said shift rail area on the T90. I tore apart a tranny and it had a small hole drilled at the back of each shift rail tube to let the oil drain back into the case rather than letting the oil be forced past the plugs in the end. Seems easier and cheaper than having a machine shop cut a groove like someone mentioned. Your thoughts? I never took the shift tower completely apart. Anything to be wary of ? Could I drill the holes without taking everything apart but just pulling the assembly off the top of the trans, drilling, deburring and cleaning up the removed material.