(Andrew, thanks for making a bug, and apologies for not reporting this
in a timely fashion.)

Although an in-memory caching solution might seem to be sufficient, if
one considers things like CGI programs, it's clear that such programs
aren't going to benefit from such a solution. It would be interesting to
know what widely deployed software does use the affected parsers,
though. A Google code search might be helpful.

I think that the nicest compatible solution would be to have some kind
of filesystem cache for the downloaded resources, but I don't recall any
standard library caching solution of this nature. Things like being able
to write to a known directory, perhaps using the temporary file APIs
which should work even as a "very unprivileged" user, would be useful
properties of such a solution.