Pieter responded to my thread. He’s worked on compressed sorted sets recently, and while doing that he fixed the issue I was having with intsets. His code should be merged into 2.2 in a few weeks after enough people have tested. Apparently I’m the only one to report the problem with intsets.
His branch’s code is similar to my approach, in that he also extended zsetopsrc with type and encoding variables. But he took it further with a set of iterator functions that work with zsets and both set encodings. Very nice!