I think the problem is more likely that too many guys (not specifically you) who usually ask for this, escalate things far too quickly to the more extreme fetishes, usually within the same 1 hr session.
So, SP's probably figure it's too much of a hassle and I wouldn't be surprised if some SP's use this as a weeding out tool.
I remembered a long time ago, Twizzler or Lurker, giving a review of an OPG girl who would do roleplay but I'm possitive that she is long gone now.
Your best bet is to see an SP fairly regularly and built up a trust level. Then when you ask the question she will at least be move comfortable with the idea.