I've researched it a bit and see that masking can be done with a php. file in a way like your url specifying
http://yourdomain.com/ebay.php but that would only work if you do every url manually. What I wonder is if it couldn't be worked into your modification with instead of adding the rover link in front of the ebay url that you add a link to a php file to the ebay url that will redirect using the ePN information but mask it to anyone else because they'll only see the php file source in front of the ebay url.
I don't think it'd be too hard to make the mod automatically add a link to a php file on the server in front of all ebay url's but the thing that I can't firgure out yet is how to make a php file redirect using the ePN credentials so we get credit for the transaction.
Maybe it could be done like your vbAnonymizer where Ebay links posted would be redirected through a php. file that attaches the affiliate info. to the url without exposing any AID's.
I don't know for sure though as I'm still researching how masking works. I just wanted to throw it out there because I see it could be useful for many of the affiliates.