There is nothing specific to MongoDB here. Also, using appropriate cache headers and putting a Varnish cache in front of the Rack server would be far easier and faster and would also reduce the load on the Rack server itself. Oh, but then I couldn't say I created a DSL...
Calling it a DSL is bit weird to me, it's a single instance method. You'd use the same kind of thing to set the appropriate caching headers for Varnish.