We are going to be implementing onion-location headers on our websites to take advantage of a new feature we will introduce into Tor Browser in an upcoming alpha release to redirect visitors using tor-browser to the .onion address. We should coordinate this work to happen on websites at the same time as we release Tor Browser Alpha and also to make a nice announcement about it. We would also like to reach out to some orgs who are operating onion services to let them know about this new feature.
I would not expect breakage to occur due to a new header clients don't recognize, but I don't see any downside to deploying this early, before Tor Browser finishes it, just in case it does expose some weird client behavior...
-tom