Forgive me if I'm wrong, but can't you just add the relevant parameters to the feed url?
I did the following for the UK feed url:
Goto [Joomla Root]->modules->mod_ebaybids->helper.php
Under 'case "uk":' Change:
$url = "http://rss.api.ebay.com/ws/rssapi?FeedName=SearchResults&siteId=3&language=en-GB&output=RSS20&from=R40&fsop=".$order."&fsoo=".$order."&from=R40&satitle=".$keyword."&sbrsrt=d".$seller_filter;
To:
$url = "http://rss.api.ebay.com/ws/rssapi?FeedName=SearchResults&siteId=3&language=en-GB&output=RSS20&from=R40&fsop=".$order."&fsoo=".$order."&from=R40&satitle=".$keyword."&sbrsrt=d".$seller_filter."&saaff=afepn&afepn=[CampaignId]";
Replace [CampaignId] above with your actual EPN campaign id (without square brackets).
This change might work for the other country specific url's also (ie adding &saaff=afepn&afepn=[CampaignId] at the end of the url).
It all seems to work, but I haven't fully tested yet.
Anyone care to try?
Steve