> How is anyone going to understand the consequences unless someone actually implements it in a stack (behind a feature-flag) that people are then encouraged to go play with—exactly as is being done here with TLS 1.3 itself?

If you read the issue that I linked, it's already been tested and a number of flaws found. It has also been tested by others who've also verified the issue with potential replay attacks. Putting something that is known to already have flaws behind a feature flag so others can, potentially unknowingly, start using it doesn't seem like a useful exercise to me and does little to gain additional data on the matter. Once the existing issues are resolved it might be more interesting to try what you're suggesting.