It'd throw off your members' last-read indicators if you start messing with what's read and what's unread.
For example, if my last-read post is #10000 and
you post #10001 and
RSS posts #10002, then when I return tonight
I'll miss #10001 completely if you're marking #10002 as read.
Better off to leave the last-read alone, and drop the RSS feeds into a dedicated forum. You can then edit the Navbar template links to Today's Posts and New Posts to exclude that particular forum.
http://yoursite.com/forums/search.ph...ily&exclude=32 would exclude Forum #32 when running a Today's Posts search.
http://yoursite.com/forums/search.ph...new&exclude=89 would exclude Forum #89 when running a New Posts search.