I don’t know if anyone’s seen this great invention for the harmonica by Jim Antaki, which has potential for use with a melodica. Should be of particular interest to Lowboy.
There’s a small magnet for each reed, which slides into place, above the reed, causing it to sound a semitone lower. You can easily slide between pitches. This would particularly suit a clavietta because of its ‘back to front’ reed arrangement, and its steel (magnetic) reeds.
Photo from Kinya Pollard’s great review article on the TurboSlide Harmonica, which uses this technology