I'm also interested in this flexibility in RSS export. It's not possible in eZ Publish as of 4.0beta1 and I couldn't find any contributions for it. I think this feature should be reported as an enhancement request.
Today I just got an idea how to have better control over RSS, it could be done using special pagelayout for RSS. It needs knowledge and understanding of RSS system, but I think it's not a big problem to figure that out. I'm going to work on it and when I get some nice result I'll publish it as a contribution.
Of course, unless the eZ crew creates some other (more generic) soluution for this issue.
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Nothing is impossible. Not if you can imagine it!
I use a special layout for RSS (and others) exports. it works well. I wrote an article about that (in french) : http://pwet.fr/blog/des_fils_rss_sur_mesure_dans_ez_publish
