This file describing users’ profile metadata includes a field for “lud16,” which would be a Lightning Address. So maybe it’s possible to set that through a Mastodon client?
https://gitlab.com/soapbox-pub/mostr/-/blob/develop/src/nostr/schema.ts
I think they’re talking about ActivityPub keys there, but it looks like your Nostr private key is derived from your ActivityPub ID plus some secret in a configuration file. Every time you post a note to Nostr, Mostr regenerates your key to sign it. There’s no API to just return your private key.
https://gitlab.com/soapbox-pub/mostr/-/blob/develop/src/nostr/sign.ts
Discussion
nostr:npub1g0wmxtcsjnnmq7x2vdpymczss47ch5gzuwfkw47s22lehvklxa2scpuvzk As far as I can tell, I can only set anything like that in the profile metadata, where I can interact with a table where I can make custom labels and content (which I only use right now to show my pronouns so dudes stop sending me dick pics). I can put whatever I want in there, but I don't quite understand if that information can be utilized by Nostr if entered that way.
I’m only a little familiar with Mastodon, and I haven’t quite figured out how Mostr fits in. Maybe try adding your Lightning Address to your profile with a “lud16” label? It probably won’t work, but I don’t know what else to try.