Define good idea.
Wouldn't this concept be better addressed by asking an actual agency? Are you asking for a real, skilled massage, or a 'pretend' one? Neither one counts as asking for sex, so there's really nothing to deter an agency from discussing it with you, if you can figure out what exactly you want. But what do you mean by "instead"; not even an HJ like at a spa? That's sex for money, which they don't discuss. The only sticking point I can see beyond that, is you assuring them that she'll be paid exactly what she would otherwise.
But I'd say the best idea is simply to ask the agency for a massage-ready/skilled companion, because that's what you like and leave all the 'instead' stuff to you and her. Still up to you to get across what you mean by 'massage' so they'll find you the girl—or admit they cannot—but way simpler.