I thought there was not such a standard, which is why people have implemented the automagic gif hack, which is what we use for crossposting.

The other problem is that, depending on the feedreader, every change to an entry may create a bump, either pushing the entry up in prominence or multiplying its presence in your reader. See ew_tvwatch_feed for a look at what LJ/DW style software does when the same entries are constantly being re-named or re-tagged or whatever. It's pretty ugly.