I got this only once. It was at Leonel, by a pretty good looking girl - might have been Mila. Total bitch afterwards - gave her $20, but she wanted $40. I said what for and she said because "I stuck my finger somewhere". I gave it too her but I haven't been back...
Also, a girl at Eastar gave me a good rubbing with pressure, but no insertion. Very limited options available in general a this place, so I suggest you look elsewhere.
Let us know what you find. I too am interested. I've even considered going to a dominatrix that offers dildo play, but this may be a little much!
I suspect a lot of guys either don't enjoy this or are affraid to write about it.
The best thing I've experienced is a rimjob by my GF, but I would never do that with an SP even though it is available out there by some.