It should be possible to leave the reed plates in the instrument while tuning. We’d need a custom attachment for Alan’s hose. After taking the keys off, there will be access to all the apertures normally covered by the key. The attachment could have a rubber gasket, and be small enough to fit neatly around one hole. The suction should hold it firmly in place.
Like a doctor checking with a stethoscope, one could visit each hole to check the reed’s rate of vibration!
An attachment like this would be easy to design as a 3D print. I made something similar to be placed directly on a reed plate, where you blow through a tube to access individual reeds (from above). That technique didn’t work though, the reeds didn’t sound. This could well be due to the direct and unregulated air pressure that you’re talking about MM.
Here’s an interesting little manual accordion reed tuning jig – the only ones I’ve seen before a bigger, with foot pumps:
It might be possible to make something like this where you slide the whole melodica along (minus keys) to access different reeds.