Well, I finally found info on the web about how exactly the e-reader is supposed to work with the GBA-SP. See the article at http://pocket.ign.com/articles/382/382062p1.html for more details. Basically, the connector port that plugs from the e-reader into the GBA is nothing more than a pass-thru that allows gamers to connect a e-reader-plugged GBA to the GameCube. It has no other function, which means that you just plug the e-reader into the GBA-SP's cart port (with the connector port not connected to anything), and the e-reader will work just fine.
Now, why is it that Nintendo hasn't just taken two seconds to tell that to everyone, instead of making people wonder how the heck the e-reader was going to work on the GBA-SP?