Yes, it's fairly practical. We use Nostr to relay Bitcoin PSBTs and other metadata about Bitcoin multisig wallets. We operate our own relay (strfry) and set a filter so it only relays our known message kinds.

It's open source: https://github.com/smartvaults/smartvaults

We have a vault template for inheritance that is configured with an absolute time lock (unlocks on a specific day) and a set of keys that can recover in the case of death. If the user is lucky enough to live past the time lock, they create another one before the first expires.

I think there's an enormous opportunity in an "open network" of cosigners. Co-signers-as-a-service can offer their services and users can decide which cosigners to subscribe to based on security, speed, trust, and other criteria.

Unchained recently launched a "enterprise custody network" where they choose the cosigners. The Smart Vaults network can achieve similar outcomes without the counterparty in the middle deciding who can and can't serve as a cosigner.

Smart Vaults are miniscript-based, P2TR Bitcoin wallets that use Nostr to coordinate metadata and PSBTs. Users create a single “account” (mnemonic) with the mobile app, and that key is used for both Nostr events and Bitcoin transactions.

You can invite other users to participate in collaborative custody vaults.

There are vault templates for decaying multisig, social recovery, and others, and there’s also an advanced miniscript policy builder using drag-and-drop shapes.

Since we use P2TR, we can technically support multisig wallets up to hundreds (prob 999) signers. When we launch the main net app, we want to set the record for the World’s Largest Human Multisig and submit the results to Guinness.

The desktop app and SDKs are open source : https://github.com/smartvaults/smartvaults

Join the Smart Vaults telegram channel to share npubs or for support here: https://t.me/+I3B8_4tz7sMwZjVh