Well, I looked around in my attic and I found an old electronic device that had the same kind of connector that'll mechanically disconnect the + lead from another circuit when you insert the plug.
I did buy a new diode from RadioShack, but what I found was that they're not 100% after testing it with a 9V battery. With no diode, the 9V battery read 10 Volts on the DMM. With the diode in place and in the proper direction, it read 9.43 Volts. So, a little resistance is added, and given how quickly the batteries are depleted in a TE, it'd be a bad idea to use it...
And finally, flipping the diode resulted in a .43-1.43 Volt measurement... so it doesn't even accomplish what was needed, I would still have to remember to remove the batteries before using the external power supply!
Anyhow, thanks for the info PMs! This was a fun little project!
BTW, I broke in the capacitors fully today, played a full game of Super Star Soldier and got to the last stage ( 8 ) only to get my ass handed to me... Interesting false alarm in the beginning though: in the 1st stage, the game kept crashing half way in, so I thought maybe something didn't quite go right with the cap replacement, but I took out the HuCard, cleaned it up a little with glass cleaner, put it back in and everything was peachy after that, aside from me being terribly rusty at the game.
But yeah, working good now! I figured it had to be the game more so than the caps, cause Air Zonk was working fine and I got up to stage 4 on that one!