Bought two of them, neither one works worth a crap. Tried soldering a NYKO battery pack into the cover, with intent of replacing the charger internals with it, but it turned out...messy

If a charger cannot measure delta-V correctly, it'll overcharge the battery. With lithium battery, you are looking at exploding battery.
Do other wireless GCN controller use their own receivers, or share the same receiver as Wavebird?
If they use their own receivers, then you need to find out if Wavebird's receiver strips rumble data before transmitting.