Just what I've been gleaning from other Meshtastic projects and my preliminary recon. A native lightning transaction doesnt seem possible. It's gonna have to send the request to a local hub to execute the transaction and then send a response, which could be several minutes. Not terrible solution in a black out or black marketing situation. It's been many months since I looked into it so let me know if you're seeing any new developments.
Discussion
Agree lightning not possible because the "money" is in the node (assuming that node is not the mesh device), and also the tx itself is too large. Ecash may work because the "money" is the token itself. Callbacks to the mint are required but maybe the mint can be mesh connected, or maybe the callback can be async.