References:
The Best Documentation I've found on the Transfer PAK
https://github.com/joeldipops/TransferBoy/blob/master/docs/TransferPakReference.md
A few more tidbits, but very incomplete compared to the above.
https://sourceforge.net/p/nragev20/code/HEAD/tree/docs/Transfer%20Pak%20Stuff/
Everdrive 5v to 3v3 modification.
http://krikzz.com/forum/index.php?topic=1780.75
https://www.dropbox.com/s/hei9yi4618manin/E64TPMOD.pdf switch modification.
https://imgur.com/6ske7Mb Soldered modification.