Hmm... a few possibilities (none of which I've tried myself, just brainstorming)
1) mp3gain.exe (the "Back end" half of mp3gain) is a command-line program that does all the real number crunching and file modifications. You could probably work up some sort of batch file that first calls your encoder (I hope you're using LAME) and then immediately calls mp3gain. Then in EAC, in the Compression options, go to External Compression and put in this batch file as your user-defined compression program.
2) If Andre really does incoporate something directly into EAC, he'd probably be better off simply incorporating the Replay Gain algorithm to analyze the raw data as it comes in. That would be simpler than reading the raw data, encoding to mp3, and then having mp3gain decode back to raw data for processing it through Replay Gain (which is what mp3gain does internally for "Analysis").
The Replay Gain algorithm is included in the mp3gain back end source code at
http://www.geocities.com/mp3gain
Just some thoughts...
-Glen