Some updates:
I've refilled stocks for sowing seeds. That should give plenty of plants to work with.
Still not decided on what media might give the best results. Maybe going S/H would work?
I'm thinking that if the potting mix doesn't break down, it's less variables.
Less unknown bacteria popping in to break the media down, less stress and potential damaged caused by re-potting, more consistency in the watering schedule.
Plus, something like pumice has lots of surface for hosting bacteria! And high humidity would be nice for beneficial fungi to stay alive.
I'm probably going to do the experiment with Phal species because it's what I know best and have several spikes going on. But if you have a suggestion for a test subject, feel free to say it