The patches available here add four configuration directives to mod_proxy that allow headers and expiration times to be set on a per-uri basis.
The changes are available as a patch file for the Apache 1.3.26 source. There is also a documentation patch for the mod_proxy.html page. (And the original version of this patch, for Apache 1.3.19, is also available.)
To apply the patchfile, cd to the src/modules/proxy/ directory and do a
patch < patchfilewhich should patch five files. Following which, it should be possible to build Apache as normal (or just build mod_proxy as an shared library and copy the .so file to the right place).