Btw I think sooner or later they will be possible via pure LN (+ send optional proof via nostr).

It's not possible now because when paying, the preimage (proof of payment) is only available in the LN wallet, which is a different app than the nostr client.

As LN SDKs get better, it will be easier to integrate a LN wallet in a nostr app, which will solve this problem at its root.