I don't see any reason why it wouldn't be syn free - you're not cooking, blending or pureeing the banana. I've never tried frozen banana myself but I know some people have it so they'll be able to tell you if it's any good.
I love frozen bananas. I slice them and always think they taste like really posh banana icecream Haven't had any since I re-started SW 2 weeks ago, I need to pop some into the freezer.