The development team discussions led to a suggestion:
We have this:
void CRSAPI getUserAssignableParamGroups(ElementIdArr &arr);
Ha! Which is exposed:
bool Revit::DB::FamilyManager::IsUserAssignableParameterGroup(
Autodesk::Revit::DB::BuiltInParameterGroup parameterGroup)And documented:
FamilyManager.IsUserAssignableParameterGroup Method
Harry, did you try this one?