A headers frame before content is just equivalent so „sending headers“, since Http/2 also only allows to send headers once per Stream unless those are informational headers. In the same fashion, sending a headers frame after content is equivalent to „sending trailing headers“ - which are allowed to be sent at most once after the body (which may be empty).

Therefore the fact that there is a frame involved doesn’t really matter.

HTTP/3 doesn’t change the HTTP semantics: Peers are sending 0-N informational headers, 1 set of request headers, a stream of body data, and 0-1 set of trailing headers. Therefore gRPC should run fine over it at long as the underlying HTTP library exposes all those necessary features.