Here's another idea for true "on the fly" instant tuning swaps whilst playing, which Dominick is really after. It involves adding an extra reed to the appropriate comb chamber (hole 3 in his case, to swap between Richter and Paddy-Richter - G and A on a C harp), then add a muting mechanism - basically a variation on what Roman proposed earlier.
It's not simple, but for a trained engineer like Dominick with good machine tools, it should be achievable. I haven't done it myself, but here's how I picture the process:
First, widen the comb chamber behind the mouthpiece so an extra reed beside the existing 3 blow reed can be added. With Dominick's design and 3d printing skills, this will be easy - but it could be done by anyone through modifying an existing comb. Then, instead of cutting a new precision reed slot in the existing reedplate (which would be tricky and time-consuming), simply cut a relief slot about 3.5mm wide, at least 1.5mm to the left or right adjacent to the 3 blow slot. This is best done with a small milling machine, but could be achieved with Dremel-type hand tools - or even a fret saw.
Then, from another reedplate, cut out a small single modular reedplate with the correct reed pitch (the A reed on a C harp). Stick the modular reedplate to the main reedplate directly above the relief slot described above, being careful not to encroach over the existing G blow slot.
An easy way to do this is using Blu Tack, as I did back on my old 11-hole Stretch-Harps (as shown/described in the previous post). It will give a perfectly airtight seal and last indefinitely. Another easy way is to follow what they do in accordions, and use wax to seal/holed the reedplate on. Or it could be secured with a thin gasket and a couple of screws.
Now comes the hard bit...! I haven't thought through exactly how to do this, but hopefully Dominick's engineering brain will come up with a good mechanism. Some kind of instant selective reed muting system to swap between the G and the A note is needed. Possibly spring-loaded mutes mounted directly above the reeds with buttons in the top cover?
However it's tricky, because one reed is higher than the other...
If it were possible to cut an accurate reed slot in the existing reedplate to attach the A blow reed, then a slider mounted on the reedplate pushed from the right could be used to selectively mute the two reeds. That would probably be the most elegant solution, but it requires accurate slot cutting to retain good sound on the inserted reed.
It's not easy, but definitely achievable
Trying to extend the idea to several reeds as Roman wants (presumably blow and draw?) is another level of difficulty altogether. In that case, I think just get a chromatic and retune reeds as appropriate: much simpler!