In Sam Ruby's comments I cam across this proposed efficiency for Atom (and presumably RSS) feeds: mod-speedy-feed. From the description:

Best of all, the content that's sent down, while smaller, remains valid Atom XML, so no real change is needed on the client side other than sending the new headers. All you need to do on the server side is compile and install the module, it works as a filter so any content that's served up with an application/atom+xml content type is automagically effected.

So to get this working, you need:

• A patched version of Apache so that you can support a new status code
• Changes on the client side to send the new headers

Exactly what is my motivation (for a tool like BottomFeeder) to support this? Only a handful of servers are going to support this in the short term - and, unless the Apache Foundation decides it's a good thing, only a handful of servers will ever support it. In other words - I have no motivation.

Here's another question though - what real benefits does this approach offer over mod-gzip? Most tools already support that, it works with arbitrary (Apache and others) servers, and XML text compresses remarkably well. Sometimes the flood of strange ideas is just amazing...