Briar does something like this, tried it once, was a little rough around the edges, but it worked.
Discussion
Yes true, Briar is also able to do this and has worked over bluetooth for years. I would seriously prefer Briar over Bitchat for this internet-is-down use case.
I'm all for multiple options, but I'd agree in this case, Briar has been used in the field for years, and has all the necessary features. It's the only offline messenger that's ready to go right now.