I was in a similar situation to you a few years back. I had never given a facial, but my then fiancee/current ex was totally ok with me cumming on her tits, and a few 'misses' had hit her face as well, no complaints.

One evening, we'd both had a fe drinks and were talking about sexy things, and turn on's and I told her how hot I though facials were and how turned on the thought of giving one made me. She got "the look" and started rubbing my crotch a bit as she unzipped my pants and gave me one of the best blow jobs I've ever had.
At the end, she looked up at me and started talking dirty, telling me how badly she wanted me to cum all over her. Instant pop and it was enormous.

From then on, she'd do that every so often as a surprise or just for fun and it was an amazing thing.

So, just tell her how hot you think it is. Tell her how sexy she'll look. Most women are far less opposed to it than you think. Since that relationship, I've had girls who would take a facial in lieu of swallowing, and better than 70% who either really enjoyed it, or did it for laughs.