Honestly? I have a feeling this is going to earn me some thrown produce.
I don't think you have to tell her anything. You have stated that should you feel you need her you will call her. You are debating whether or not, in the event of a UC, you'll call her afterwards as well. It's not like you hired her knowing you never have any intention of using her. You have admitted the possibility is still open. Only difference between you and the average homebirther is you are planning with a different degree of certainty of UCing. Even if you knew 100% you were going to call her you could not guarentee 100% that she would make it. Your "be prepared for it" instance is just a more planned out instance. You've just made the concious decision (well almost ) as to whether or not to call her because you are comfy with either or in this case. Hope that makes sense.
So long as she gets paid I don't see why you have to divulge any extra info. It's not like she could guarentee making it to a birth for a non-UCing client. And if she didn't I doubt she'd flip her top. Is it sneaky not telling her upfront? No, I don't think so. You know she's not UC friendly so you're protecting that part of your birth space. You're not an idiot though. You know you hired her and you know your reasons why. You are still considering calling her in the even of needing her. Sounds like most homebirths to me. Just with a twist in the direction of being a confident UCer.
Ok, throw your produce. I have my salad shooter ready.