Is it possible to import a rigged mesh with constraints and drivers from Blender to CC4 as CC_GCharacter?
Blender’s constraints and drivers are native to blender internal system and cannot be exported to external programs AFAIK.
If not, is it possible to add bones with custom constraints/drivers (or whatever they would call it) in CC4 or any of the other Reallusion products?
I doubt you could achieve any custom rig adjustments to a CC4 character as was shown in the video you linked
the Iclone/CC4 Avatar rigs have very limited editibility from what I have seen.