Because there is no past perfect continuous passive in the English Grammar Profile, we can say it is C2.
A search in NOW corpus shows that this grammar is extremely rare.
1 HAD BEEN BEING TREATED 10
The woman had left the Ulster Hospital, Dundonald, where she had been being treated.
2 HAD BEEN BEING ABUSED 7
3 ‘D BEEN BEING ABUSED 7
4 HAD BEEN BEING USED 7
5 HAD BEEN BEING TAKEN 4