Those APIs (i.e. fetch) are regular async/promise based. And in this synthetic scenario you can't construct your response-to-caller without something in that respond payload. I don't see a way to solve this all with magic.
Any code sending an outbound request in reaction to a write is causally related and could be represented as a pipelined promise. The receiving system can then proceed in its work until it needs to "await" the incoming promise and can see whether it was broken due to a failure to persist some earlier write. This could also be handled at the network layer if the receiving system was external.
I'm pretty sure I remember Kenton announcing that Cloudflare Workers now supports something like (or exactly) object capabilities and promise pipelining and his knowledge/interest in such systems is already reflected in Cap'n Proto RPC.
Very cool stuff if you ask me!
At present output gates operate on the scope of a single worker, blocking the output from being sent. It's easy to imagine, though, that we extend things so if you are sending a message to another worker (including a Durable Object), the message is sent immediately, but the destination worker becomes subject to the same output gate. Haven't done it yet but would definitely like to!