Can PC be used on the roll that determines how many clicks of healing a support fig does?

Yep! Remember, it's a roll that you would make on the turn of the person using support. Therefore, the one performing the support roll would be allowed to prob their support roll, as would his/her opponent.

On your turn, you can prob all of YOUR rolls. On their turn, you can prob all of THEIR rolls. So things like Super Senses and Impervious can't be probbed for except for some very strange circumstances, but hopefully you get the idea.

Long explanation short, you can use PC on a support roll. And it's quite useful for that as well