I threw together a quick script to integrate Agenda with LaunchBar: it lets me open a note by activating LaunchBar and typing part of its title. It probably also works with Alfred and similar; all it does is fill a folder with .inetloc files, each of which points to an Agenda note. You’re welcome to give it a try:
It’s a bit ugly that I had to go spelunking in one of Agenda’s private sqlite DBs to get the info out, but it shouldn’t be too hard to adjust as the schema changes. It’s a simple hack for now, but I’ll round off the rough edges if I end up using it longer term. Cheers!