Just recently I was in central london on a busy saturday night and finding a toilet was seriously hard work. Macdonalds had long queues for toilets, so did many other retail outlets and many tube stations dont have toilets. I know residential areas are a little further out from the city, but i'm sure people could get creative and make it work.
I'd even check the toilet "spots" before I went to a crowded area so I know i could pop in when needed.
On the flip-side, acting as a host however, I dont think I could do it. With the state of toilets in most service stations, I couldn't give up my toilet to someone to come in and pee all over the seat or forget to flush or something.
The irony is that, that's going to be the real sticking point.