This might interested you. It’s not exactly what you’re asking for but it might achieve the same thing. This discussion is in the context of HTTP requests.
I was thinking of a guaranteed way of calling an update after returning the fast query. I want the client to get the query response quickly, and the I want to record an Incerement of a served asset in an update. I could have the client do it, but it would be hackable. If the service worker did it as part of the protocol it would be cool and allow us to do gated assets.
That seems doable but with the caveat that once the client making the HTTP request gets a response (the result of the query) it has no way of being informed of the result of the update call (success, failure, etc.)
For use cases like the one you mentioned that might be fine.
I think this whole feature deserves a proper specification. One may even exist somewhere in private. @nomeata might know more.