I've just spent the weekend tuning brass reeds from an organ. It sounds like a very similar process, except you can grind both ends of the tongue to raise and lower frequency.
And, if you're sneaky, you can add solder.
Adding solder has also been frequently used to correct the resonance frequency of quartz crystals that have been ground too much, and I mean during industrial mass-production, not only in a home-lab setting.
Oh that is so cool. I played the one in Liepaja, Latvia for a bit and it was absolutely amazing. It's love/hate for me (like the harpsichord), I love the instruments but I usually do not like the music that is played on them because of the grating effect. I have pretty bad tinnitus which really spoils a lot of music for me, extremely annoying.