yes you can have a keyshare held by a custodian, and have that custodian participate in signing

it's the same model as 2/3 with bitkey, casa or unchained, or any other multi-sig-as-a-service solution

we are developing a server-less API gateway that people can run themselves, or a professional custodian can run on behalf of their customers

that being said, you are better off running your own server, and handing out revocable tokens. remember that 2/3 with custodians is a compromise to help on-board normies away from single-sig setups, and should not be a desirable long-term solution

nostr literally fixes the need for inviting custodians into your quorum. just run a gateway node on a laptop under the bed, don't give up *any* of your keys to a custodian. this goes for all multi-sig setups period.

I'm running permafrost + frost2x, and soon(tm) heimdall for permissioned API access to my key