After patching CompoundFunction to support variable number of independent coordinate everything appears to be working as expected. I guess I'll create a PR.

The OpenSim::CoordinateCouplerConstraint appears to support multiple independent coordinates, however, when I actually tried to use this I get an assertion error: Constraint::CoordinateCouplerImpl::CoordinateCouplerImpl (SimbodyMatterSubsystem& matter, const Function* function, const Array_<Mobilize...