Personally, I don't see the point in criticizing other people's sexual behaviour.
As far as your question goes, if you're already lying there naked with a hardon, paying a sex trade worker for sexual services, and you know you'd much rather get fucked than settle for a handjob, there is no harm in asking politely.
This is not inappropriate behaviour. She is a sex trade worker doing her job. In this context, all you're doing is clarifying the boundaries. It's not the same as making a pass at your secretary.
This is the win-win option. If it is on the menu, you get what you want, and she makes more money. If you don't ask, you are guaranteeing that you don't get what you want. That would be the lose-lose option.
The only risk is that she will say "sorry, that's not on the menu". Not a big deal. Getting turned down by an MPA is not the end of the world. You'll be over it before she even gets started on the handjob. It's not the same as getting shot down and laughed at by that hot chick you had a crush on in highschool.