update:
N64 wireless controller is finished, works great!

Some pics I made:



Essentially, modding the controller means routing the data pin to the memory card port and installing a LED, that's all.
The rest of the circuit is inside the rumble pack.
I also did some measurements. It turned out that the N64 controller is quite power hungry: The wireless circuit draws only 2 mA, but the N64 controller itself draws about 14 mA!!
Under normal circumstances I would cut the N64 controller's power and only turn it on when I want to read the current status. Unfortunately if I would do that, the N64 controller would reset the analog stick everytime I'd turn off the power.
So the controller needs to be turned on the whole time.
16 mA is no problem when you're using 2x AAA (rechargeable or disposable) batteries within the rumble pack, because the capacity of such batteries lies between 800 and 1000 mAh usually. This should last a while

You could also install a large (800++ mAh) LiPo battery inside the rumble pack, there would be plenty of space if the battery compartment is removed by using a Dremel.
An all-internal mod inside the N64 controller is possible, too. But you'd need to find a battery with a decent capacity which still fits inside.
Oh, and before I forget: There won't be rumble & memory card support! So you might just as well use the space inside a memory card or rumble pack for big batteries
