Alec Muffett:
I have little useful opinion to add to this right now, but there is already an existing draft for an "Alternate Service" header - which may be relevant or inspirational:
https://tools.ietf.org/html/draft-ietf-httpbis-alt-svc-09
At scale it might be costly to issue a header to all browsers in order to advertise an alternate service to some small fraction of people whom are actually able to (or desire) to use it.
Martijn Grooten:
On Thu, Feb 04, 2016 at 03:36:44PM +0000, Alec Muffett wrote:
Perhaps only issuing the header to people who access from an exit node, might reduce that cost?
Even so, and especially then, this sound like an easy way for someone operating a rogue exit node to get persistent MitM on non-HTTPS sites.
Could onion services be announced via ALPN maybe?