I too would much appreciate just intonation for the Interval Loader. Ideally one should be able to switch between equal temperament and just intonation, and then select key for just intonation. The advantages as I see it,
People with natural absolute pitch use just intonation
The harmony is better so it is easier to learn and hear when it in tune
Equal temperament is just a compromise in order not to have to retune the piano when changing key, nobody really think it sounds better
The differences between different keys is emphasized with just intonation
Lot's of classical works were written for just intonation (or similar temperaments), if you don't practice the various keys in just intonation you can't appreciate the choice of key for the classical works
Interesting ideas here. And I like the idea of being able to listen to just vs. equal temperament- both do have pros and cons though. Personally I think it would be easier on the user if they used the same tuning that their instrument does, for if they develop AP, it's more likely to be in the same tuning system than their instrument is in. For me, for example, my AP seems to default to 12-tone equal temperament since I grew up around a piano. However, just tuning works for me as well. There is a subtle difference between the two, certainly. The whole issue of blending with others and using just tuning was interesting when I joined a high-level chorus and got to sing perfect intervals against my choirmates, as well as blend when they weren't 100% on. Point being, that one tends to develop AP according to what tuning system they use. So for example, for a vocalist developing AP, using just intonation in APA could be excellent, while for a pianist equal temperament may be of more use (at least in the beginning).
Also, keys are identifiable and characterized based on chroma. Sure, just tuning may bring out some aspects of keys more and make other keys sound quite interesting, it's really chroma that characterizes keys. I feel a lot of composers (or at least I can vouch for myself as a composer) choose their key based on this chroma, although another source of key choice is practicality. For example, D flat major is not a practical key for stringed instruments to play in.
All in all though, this idea of a button for temperament does sound like an interesting idea. I'm intrigued by it and look forward to seeing what happens in this area of APA.