If mod_transform isn't supporting Etag properly, then I'd say mod_transform was broken, and fixing it would probably solve your problem.
+1. If the content changes, so should the ETag. mod_transform could also set some Cache-Control headers.
In short, think about what an intermediary caching HTTP proxy would do with that request. It'd check the expiration header ('freshness' tests), and, if that fails, then the external cache would try to send an If-Modified-Since (or some variant) request to the upstream server: if httpd responds it hasn't changed, then it'd serve from the cache until the timeout. So, it'd exactly act the same as mod_cache. Hence, adding 'private' hooks for mod_cache would still allow stale responses from external HTTP caches. -- justin
