I think I've decided my course of action. I agree with AussieLover that I really need to find out what F is thinking and how she's feeling. I know I'm not going to get an honest answer out of her if I come up to her cold and just flat-out ask, so I've started flirting with her again. When and if I can get her back into flirting with me (which, if she won't, I know is pretty indicative), then I'll try to find a moment when I can talk to her alone and ask. This is going to be the difficult part, since I so rarely see her without my wife. From there, it depends on her answers, of course. If nay, I suffer in silence, hoping the desire will subside; if yea, then we discuss the ramifications on our spouses, and go from there.
Halloween went very well, though, as our group all get together to party and everyone dresses up. F was Bellatrix from Harry Potter, and kept driving me wild doing the Death-Eater thing and licking her 'tattoo'. When picture-taking time came around, she wrapped herself pretty close to me and I thought I was gonna lose my mind on the spot.
My wife encourages me to tell F that I think she's pretty because F's overweight and sensitive about it. I told F she looked beautiful that night. She always emails a thank-you note after we all get together, and she said how she loved our costumes, that my wife and I both are good for her ego. I reiterated that I thought she was gorgeous, and good for our egos as well. She thanked me for all the compliments.
Everyone will be out again Sunday, so I hope all goes well. Thank you, everyone, for giving me the peace of knowing I should seek the truth and the encouragement I needed to do it. I'll keep you posted!