Are coarse 3D mesh seat pads effective at keeping your butt cool on a recumbent?
Thinking about my stock seat pad - I have a stiff plasticky mesh inside the seat pad, which has a layer of cloth on both sides.  The mesh is an irregular plastic weave, and would be uncomfortable to sir on without a cover.

Thing is, to get the seat back sweaty I've already sweated through every layer of clothing on my back, and lower.  The cloth layer on the seat pad is also between me and the ventilated area.

So I feel that a combination of less clothing on the rider plus more airflow under would help.

Replacing your foam squab with something more open certainly is a good idea.  I'm contemplating something with ribs that run vertically to allow airflow through.

Something vaguely like this backpack design, to keep your weight off your spine, possibly with 2-4 pool noodles or similar: