Okay, so from my tinkering I'm just gonna post some of the odds and ends as I learn them about this setup and the games that support it.
So, first off, how to get the thing to work:
You can't just plug the interface cable into to Barcode Battler II and the 2nd player controller port and turn it on. When I first tried it out I did this, and I got nothing. So, I started going through the instruction manual for Excite Stage '95. I learned that you have to hold down the R-Power and R-Battle buttons while turning on the power of the Barcode Battler AFTER setting it all up.
In Excite Stage '95, there are barcodes printed on the outer edges of several of the back pages of the manual. You have to go into the game's options by the picture of the Barcode Battler and select that yes, you will be using one. Afterward, choosing your mode and team will allow you in your roster setup to scan barcodes. The ones in the manual give high-ranked players for various positions, so having the manual is desirable if you plan to use the Barcode Battler features. My next experiment will be scanning in other barcodes to see what they yield.
In the Conveni Wars, one of your main menu options is simply a graphical interface for your regular Barcode Battler vs. game. It gives a Pokémon (or closer, Bio Motor Unitron)-like battle interface, and both sides are controlled by the P1 controller. It's a novelty, but doesn't offer a particularly new experience. Next on this will be attempting to scan robots into the grid-based strategy portion of the game.
I've also ordered the Lupin III game and the Doraemon games, and am looking for a set of cards for my Excite Stage '94, as apparently there was a custom set for that game that didn't come with mine.