Last-Modified / If-Modified-Since / 304 support with data-based timestamp?

[jensb]

There is a PR implementing Last-Modified header support, but - if I have read this correctly - this will still regenerate the page content to check whether to retransfer it to the client, just like ETags do.
I'd like to be able to pass a manual timestamp to an action's Last-Modified header, gotten via a (fast & cheap) DB query, instead of starting the normal (expensive & slow) controller action whose result is supposed to be cached.
Is this possible using this library? The crucial question is probably how to get the timestamp value out of the database even before starting the controller action ...