It's just the case that you can't upgrade from a HTTP connect/request to a websocket connection, since at that point the connection is already in HTTP/2 mode and both HTTP/2 and websockets need complete control over the TCP stream.
But: You can still make a HTTP/1 request to the server and upgrade to websockets. Both servers and clients will speak HTTP/1 in parallel to HTTP/2 for a loooong time - most likely forever.
That said, lattice is completely event-driven. The #on_event and #emit_event methods act as entry and exit points. As long as I can have string come and go between the server and client, HTTP2 would be fine.
This seems like a pretty good article covering it