Well, I think what we can do is something like:

http://foxfirefey.dreamwidth.org/data/rss

Gives the user's default cut level, whether at title, summary, cut, or full.

http://foxfirefey.dreamwidth.org/data/rss?level=full

Gives the user's feed at full. And other levels, too:

http://foxfirefey.dreamwidth.org/data/rss?level=title
http://foxfirefey.dreamwidth.org/data/rss?level=summary
http://foxfirefey.dreamwidth.org/data/rss?level=cut

Now, since some users DON'T want their full content syndicated for various reasons, we can have two settings:

* The default syndication level of your feed
* The maximum syndication level of your feed

The defaults will be the cut for the default syndication level and full for the maximum.

Now, you won't be able to syndicate the full content of somebody who has explicitly turned that off. But, that is their choice and you would have to work it out with them. The defaults would be kind to you and let you have what you need without getting in the way of others. These arguments would, of course, work just fine with authenticated feeds as well.

I think if there's a way to generate OPML files at a certain level, you should be okay then, without messing up things like LJ needing smaller feed sizes. How does it sound to you? It would have to be run by Mark, since there could be resource limitations I am unaware of, but that's the main problem I can think of.