This could be solved without trailers by piggybacking on top of server sent events rather than requests with trailers, and terminating a stream with status metadata as a final event.
In the next web, https://web.dev/webtransport/ would be the thing to move to. It's a bit nicer under the hood than websockets, though it's also brand new.
My comment was less about a full duplex replacement though, and more "how to get something equivalent to trailers, that works in browsers today", and this would enable that.
Server sent events are just a potential payload format for response bodies, which allows to encode a potentially infinite amount of distinct messages. Trailers are a different concept since they exist outside of bodies.
If the team had decided to only use http body streams for all payload, they could totally have done that without directly using SSE encoding.
Also, IIRC gRPC supports full-duplex, which SSE does not.
WebSockets may have been a better fit, as the Chromium developers hinted on the WontFix issue linked in the post.