how do I limit web bandwidth by user?

There are various bandwidth modules. They address limiting bandwidth by ip,
by interface, or for the apache server itself, but I can't find anything
that lets me limit bandwidth by user. I'm flexible on the solution.

One thing that I have found is a way to limit bandwidth by user with
iptables. This seems a promising lead, but when I watch top as I access a
large document in a user public_html directory, the process id belongs to
apache. I guess that is to be expected, but I don't suppose iptables would
be of much benefit in that case, because it can't distinguish between the
service and the user, so I believe that I will have to find a way to do this
within apache.

If I can't limit by user, perhaps I can limit by directory, and then limit
each public_html. That would be cumbersome but effective. The next silly
question if I'm going to try that, can I get apache to read the directories
to be limited through a file? I obviously don't want to use htaccess. The
end user could modify it. (I guess root could own it and I could make it
sticky with no write permissions?)

Anyway, I'm open to suggestions.