Hi Szymon,
Indeed you are right that currently we do not allow the application/rss+xml value for the link tag. In the next release (v3.2) there will be more options to choose from when editing the heading of your pages. Currently, to add link head tag with this value you would only need to drop a RssFeed control to your page and set its NavigateUrl property to your rss feed. This control automatically adds the needed tag to your header.
Hope this would help.
Kind regards,
Vassil Daskalov
the Telerik team