Great work, ventilator! I tried this back in the A6 times but couldn't get it to work because of the many workarounds it required then.

There are several shadowmapping techniques that are compatible with pssm. It's the group of warping techniques (pssm is a splitting technique). Some examples are PSM, LiSPSM, TSM, and some more. They're all based on using a different projection, so that the shadowmap space is used optimally.

Do your split-frustums fit tightly around their chunk of the normal view frustums?

shader tutorial