Have you tried amending the soil heavily with gypsum?
Yep. I actually did a test to see if that would work by digging down into the hard clay, and pouring in a whole bag to let it dissolve. It did improve it a little bit--made it slightly more porous--but not a gamechanging amount. And that was literally a whole bag of it.
The only thing that seems to work is mulching and folding this into the soil as it breaks down. I use pure uncomposted horse manure and cheap forest mulch. I pile it deep. I have dung beetles at my place, and they really do work to funnel down into the harder soil, and the worms do the rest. But this process takes time--about three years to get a layer of good soil about 1/1/2 feet deep.
Anyway, I just spoke to a guy who supplies pumice and pearlite here, and he basically said that pearlite trumps anything and that I'm betetr off just buying a coarse grade of this in bulk. Surprisingly, it is much cheaper than pumice in bulk.
He also told me--and this is interesting--that in coir mixes, you shouldn't use anything but pearlite. Something to do with the way plants struggle to get water from the coir. He totally nixed my biochar setup, so I may have just wasted a lot of energy and time with that experiment