Me again on this topic.
I spent some time with the code of pssm and I have implemented the mods to make it work with even larger terrains like ECEF databases. I got hints from Robert and Adrian (the author) and got some inspiration from the osgEarth project. I have tested it with ECEF database and it works fine, but more tests and reports are highly welcome too.
I also created a PR for the osg master repo for review.