Thanks Edvin and Triona for your interesting suggestions. I don't want to discuss the actual note-switching mechanism here, but that's not necessary to come up with possible applications: it's the concept that matters. The main points (updated, after recent posts from Edvin) are:
1. It's activated manually, like a chromatic slider, with similar ease and speed.
2. In the standard version, all reeds in the harmonica change breath direction at once.
3. However it could be designed so only selected reeds change breath direction, the rest stay the same.
4. It works for single notes or chords, whatever you're playing at the time.
5. The note pairs in each chamber remain the same, but change breath direction.
6. The available bends and overbends stay the same in each hole, but change breath direction.
7. The standard version applies to typical single-reed diatonic harmonicas in any tuning, but it could be applied to a chromatic-style slide harmonica, in any tuning.
Making it selectable, as Edvin enquires, isn't something I'd considered. It would be possible, but would require some more thought and design work. I can see that it could be handy for getting extra chords with unaltered notes. That's a useful concept alright
I'll mull over your ideas and consider how this option could be made.
So far I have a design based on a standard 10-hole diatonic, with blow reedplate on top and draw reedplate on bottom. Edvin's suggestion for extra chording using chromatic-style reedplates would also require more design work, but it's conceptually possible. A tuning diagram would help me grasp that particular idea better. (Post here or send privately Edvin, whatever you prefer).
As for your other idea that the dual-action reed function overcomes the reversed reed direction in the top octave of Richter tuning: yes, it makes draw notes higher than blows, but the note pairs are still the same. A switch to Spiral Tuning, as you say. Is that it useful? I'm not sure
Actually I think the SlipSlider is better in that respect: with a right push, it gives you the same scale in the top octave as the middle, with all the familiar draw bends etc I would choose it instead, I think.
Randy Weinstein says he prefers blow bends to draw bends in general, and has had Seydel make him a harp with a breath pattern like the top end of Richter throughout the range. This DA function would give a player blow bends in the lower two octaves, so for Randy it's useful. But I don't know anyone else who shares his preference. And again, the SlipSlider can fulfill that function probably better.
I'll think more about a version using chromatic reedplates. One idea that pops into my head is combining the DA function with Slide-Diatonic tuning. The Slide-Diatonic duplicates notes and gives you new reed pairs, meaning you can bend on notes that you normally can't.
For example, in hole 5 of a C SD, you have CD slide-out, and DE slide-in. With half-valving, that means you can bend the draw E note, which is normally an un-bendable blow on hole 6.
Now, if you activate the DA function at the same time, the notes in hole 5 would be D draw/E blow. This means the E stays as a blow but is now bendable...
Hmm... Interesting, but is it really useful? On reflection, I don't really think so. However, it does indicate that, if you can find a good tuning for the slider notes in a chromatic, there might be some utility hidden in this idea.
It's a bit of a mind-bender though. I need another coffee...