There appear to be 2 risks to this modification, I'll add more research when I've completed and tested this mod.
The modification itself is pretty easy if you have any experience with a soldering iron. The Reference video is excellent and I suggest pausing it at 3:05 to see the connections to make.
I used a piece of wire to jumper the connections, and just cut and removed the fuse.
When playing the rumble pack has a little more "rattle" with no batteries. I suggest putting something in the battery compartment maybe a Styrofoam peanut. To give it a more professional feel there are "dummy" batteries available for approx. $0.50 each.
References:
http://s9.zetaboards.com/Nintendo_64_Forever/topic/7348160/1/