GM1 settings: can't lock video exposure?

Is there some setting so that AE Lock will work with video? I have AE Lock set to the Fn1 button, and it works perfectly for stills, but not for video. I doesn't work whether I lock exposure before starting video or try to lock it while video is rolling. Is there a setting I'm missing?




(I know I can use the Movie setting in full manual mode, that's not what I'm asking about)