Like missionary in an upright position? You're probably not finding many scenes. To be standing on your head for twenty minutes or more in a scene? That definitely would be uncomfortable. If not just on your head but on your neck, your shoulders and on your upper arms too.
Of cos not for over 20 minutes as that would be tiring, but even if it for a minute or something as i know in that Lana scene she did that for a minute.