This is an interesting proposal and I'm happy to see RIPE open up HTTP requests a bit more. I hope that as you do, the WG can share thoughts on the balance of privacy and security with measurement objectives, since that will better inform concurrent efforts.
Since the target will be an anchor which has known expected content, will these measurements include some indication of whether application-layer rewriting was detected?
Case in point – there are interesting parallels between this proposal and OONI's Header Field Manipulation Test [1]. RIPE might be interested in exploring similar services to return headers to potentially detect inflight modification, or better yet to whitelist OONI's helper services.