It depends upon the bathroom. If there is an accessible stall, I bring the stroller in with me. If not I just park the stroller in front of the stall and pee with the door open. I use the furthest stall for this to avoid people needing to walk around the stroller and or get a view of me.
If wearing him in a bjorn I could pee as usual.
At home if I was nursing and had to pee I could manage to keep the boy on me if I was wearing the my breast friend, I could still manage to pee and wipe with one hand and wash my hand, while holding him on it with the other hand.
Necessity really is the mother of invention. :)