I think OPOL would work perfectly for you. You will have at least 4 years to create an influential environment of French immersion at home, with the help of DVDs, CDs, Internet Radio, French TV via Satellite, books, etc.
Once they start school it will become more difficult, but it will always stay witht her, but you need to be firm in your approach, meaning no English at all when you speak to her.
You will get back into speaking French all the time, it's your mother tongue! You still have time to surround yourself with it again, start watching French Tv etc.
Our children are bilingual (German/English), we speak only German at home. Dd1 did brilliantly in her German GCSE in year 8 with minimum effort (meaning no lessons at school, but being taught some grammar by me, maybe 10 hours work in total). Ds will also do his GCSE in year 8, he didn't choose German at school either, as he only needs some practice writing but that's it.
Our house is full of German stuff, we have German Satelite Tv, 95%of our DVds are in German, loads and loads of books, we have a teen magazine subscription, etc.
My dd2 starts school in September, she is fluent in both languages, can translate, asks me for words in either language if she doesn't know them, and tells her teenage siblings off if they talk English at home (which they do when they're alone).
Don't waste this opportunity, you can give it a try first and see how it goes. Don't worry about confusing her, she will pick up both languages with ease.