how to write a page to the cache from a cache sweeper

I've got an expensive page which I want to store in the page cache. The
conditions under which it needs to be updated are such that it requires
use of a sweeper object, no challenge there. The wrinkle is this:
instead of simply expiring the page in the sweeper and lazily waiting
until the next user requests the page to generate it, I'd like to go
ahead and regenerate the page in the sweeper. Any tips on how I'd go
about doing that?

- donald