I think its an interesting concept.
I'm not sure in this industry its a good idea to offer pwyw in setting up the appointment though just because there will be guys that abuse it, and because escort services are inherently expensive compared to most other services out there I'm not sure you're often going to end out "up" compared to just charging your normal rate unless you are a provider who typically would be fine with getting a less expensive rate in general, because maybe then you'll get guys who will pay you a higher rate (when they get good service from you) because in general they might typically spend more as a rule.
I think an SP would be far more prudent to offer a mix as opposed to a straight pwyw model if they choose to experiment like this. Put in your advertising a base rate - which is cheap comparatively - and explain to prospective clients that they can tip what they want, but that your expectations are that they are to tip "generously" if they feel they are getting extraordinary service. Most guys (not all mind you!) don't want to be perceived as cheap, especially if they get outstanding service and would like to come back and see that lady again. I know there are times when I've booked with a lady and her rates were actually quite inexpensive compared to most, and then they absolutely blew my mind!! I couldn't empty my pockets quick enough with a hefty tip, and a few times wished I had more on me because what I had truly wasn't paying value for the effort provided.
One time I had booked a 1 hour appointment with a new lady and I swear to you that she wouldn't let me leave - we had an incredible time. Hours later I finally had to stumble out because it was 7am and we both had to go to our day jobs. I had no expectation coming in than to go the hour and pay the hour plus a standard tip. She never asked for a penny more, however fortunately for her I had a couple more hundreds in my pocket and emptied them out - which certainly wasn't value for dollar. However she was telling me "what are you doing - you know you don't need to do that?" But I felt like I had to because even then it really wasn't enough from a professional standpoint, although I know she probably enjoyed herself as much as I did which is why she played with me for so long. So it was a win-win I think, and if I had just come to that appointment after a big win at the casino she might have gotten a $1000 from me too.
So maybe in a sense you could provide a pwyw model when you book an appointment with a client, and you haven't got anything booked after the appointment, and then if you're enjoying yourself with the client just rock his world and freely extend the session without asking for more $$ to do it and don't have any expectations for it. And what's going to happen is that some guys might not have any extra cash so it might not benefit financially then, but there will be other guys that will empty their wallets (especially because in a sense they think you like them - and its way easier to empty the wallet then!!) But only do that with guys you enjoy so that its never a lose-win situation...hopefully you can just enjoy the extra companionship yourself in the times you're not getting the extra tip.