Yet another addition to my findings: CC1 in fact will control Nuendo 12 in HUI Mode! This depends on the settings in CC1’s “Control Center” app which has to be used to invoke the mode by selecting ProTools as the target DAW. Quite logical, isn’t it? ;-D
Nevertheless, the implementation achieved that way is cumbersome, to say the least. There is no indication of selected tracks, let alone a bidirectional dependency between the selection on the screen and on the device. In short, apart from occasional fader movements, this is useless in Real Life.