Wow ZD – that’s an awkward situation to be in! I have been in somewhat similar situations a few times. You are right – there is no way to ask a sheilah and you have no choice but to “posken” for yourself. In some cases, I “poskened” one way and in others the other way, depending on the specifics of the situation.
In at least one case, I found out afterwards that I did the right thing. In others, I still don’t know.
One case that still bothers me was similar to yours. I was making a shiva call to a not-Frum family, and the father of the nifter wanted to shake hands with me. I politely said that I don’t shake hands with men. This usually goes over well, especially with Israelis. However, the problem was that the immediate relative whom I had come with had just shaken hands with him. So he looked insulted when I said that.
As soon as I left, I asked the relative why she did that. She responded that she wasn’t thinking, and she felt bad because he was a holocaust survivor and he had just lost his son.
The difference between my situation and yours is that I would have been an active participant and you would have been passive, so I would think that yours would be less problematic. On the other hand, in your case, could you have politely requested that she put her hands above your head (as opposed to on your head)?