Well I haven't personally seen it yet.
The pope once asked me to stop fighting HRE within 10 turns I think it was.
Problem is that we had fought for some time now so HRE hated me(I was the danes) and thus impossible to get a deal with.
However it seemed like HRE got the same demand before me because after 5 turns he got excommunicated.
That's about it when it comes to the pope for me.

But there are alot of factors that now play in on the diplomacy.
A screw-up can do bad things to your reputation and once that is gone then you will have a tough time getting deals.