IMHO, not so keen on this one.
Logical as there is a skirt involved, but the term upskirt is a focused action.
Its a specific thing of either an on purpose worms eye view up a skirt, or secretly taking a gander/photo, or look up a skirt.
Going by that logic, the tag anal_penetration, anal_prolapse, anal_(anything) should just be anal.
I understand the concept well of Mission Creep where you could try to be so targeted that the tag list is becoming unmanageable, so keeping some reins on them is needed,
but in this case, im going to think no.