Thank you all for your thoughtful replies.
@RyanDean and @Funguy, I think what you say about my partner setting the boundaries is important, though maybe there will be some soft limits to define. One priority for me is that my partner really is on board and into the play instead of just doing it for me. Otherwise that sounds like a recipe for no one having a good time.
I've been honest with my partner about my kinky tendencies. We did some role play once, which we both thought was a bit fun, but it was difficult for him to stay in it. If I had bad knees, it would be tough for me to focus on staying in character, too. The role play made him giggle a bit at times, which kinda ruined the mood for me. When we've tried some other kinkier things, it made my partner giggle a little bit again, too. I asked him about it, and he said it made him a little nervous at first but would be willing to try again sometime. (There are other things that have made our sex life a little difficult, including spontaneity, but bringing in a 3rd would definitely be a little bit of a wild card in terms of chemistry.)
I'm not even suggesting that this has to be a particularly kinky encounter. It would be nice to have an experienced professional help us explore our mutual needs and shift moods to suit each other. I guess I'm imagining something like a sex coach that takes a hands-on approach...literally. I really want to protect the relationship that my partner and I have grown together, but I don't imagine shaking his confidence by bringing in an escort once, or even rarely, as long as we approach it like a mutual fun, guided experience.
Thanks guys--just responding to your comments is helping me clarify my perspective, reasons and concerns. Did what I say make sense?
xo