premium podcast feed under category feed

If you want simply a category feed with posts with multiple media files then you already have it. If you category feed is http://example.com/category/categoryname/feed then the URL of the ATOM feed is http://example.com/category/categoryname/feed/atom.

That posts of ATOM feed may have more than one attached media file is a matter of the ATOM feed specification and has nothing to do with the Premium Content function of podPress.
This Premium Content function is only available for podPress Feeds and not for category feeds.

I’ve tried subscribing to http://en.meetitalia.org/category/travel-phrases/feed/atom
in iTunes but […] I just get the usual RSS2 media files.

The posts in the ATOM Feed are having more than one attached media file (see yourself and open the URL e.g. with the Firefox or the Safari browser) but while iTunes can handle ATOM feeds in general. But it shows only the media file which has been attached first to the post. That is why the result is like you would have subscribed to the RSS2 feed.
That is a limitation of iTunes.