> I guess e.g. Nginx could also insert an artificial header to mark requests received as 0-RTT, and frameworks like Django could use that header to require views be explicitly marked with a decorator to indicate support, or something like that

There is an Internet Draft for that [1]. It is co-authored by Willy Tarreau of haproxy and implemented within haproxy 1.8 [2].

[1] https://tools.ietf.org/id/draft-thomson-http-replay-01.html

[2] https://www.mail-archive.com/haproxy@formilux.org/msg28004.h... (Ctrl+F 'Early-Data') https://www.mail-archive.com/haproxy@formilux.org/msg27653.h... (Ctrl+F '0-RTT')