Way back for Nostrica I built a full signing side flow in a dev branch of SeedSigner. Fully functional airgapped nostr signing. BUT... airgapped nostr signing doesn't make a ton of sense for current social media-like nostr use cases. Part of the goal was to get some momentum behind NIP-26 to create delegated signing keys. The idea was that the only airgapped operation would be to create a new delegate, then you load the delegate into your client as the hot / at-risk key. Eventually it expires and then you generate a new one via SeedSigner.