TG-5 preset focusing distance for video?

When shooting video with the TG-5 I tend to zone-focus by presetting the focusing distance before recording. This prevents the TG-5's tendancy to hunt. However in order to do this you have to half press the shutter button and press OK at the same time, which is impossible to do on the Seafrogs housing.
Is it possible to set a custom function, or perhaps any other suggestions, on locking the focus when shooting video to prevent hunting?
TIA

Have been shooting Oly cameras underwater for many years. The tough series are fantastic and simple. 

I would try the half shutter and then hit record. A little hunting can be expected depending upon conditions and subject. If you post process, you can delete anything that is prolonged while keeping the "in focus" part of the clip.

Hi Bill. The half shutter trick doesn't work on the Seafrogs housing. Believe me, I've tried many times and end up taking a photo instead. The hunting issue on the TG-5 is well-known (the TG-4 was slightly better) and my experience of snorkeling with it while moving was always to lock focus, otherwise footage is unusable (static shots aren't so bad). I was just wondering if it's possible to preset the focusing distance and save it as a custom setting but I've tried and it doesn't seem to lock focus and save it, unless I've done something wrong.

I've just had another thought: maybe I could set the video mode to MF as a custom setting. That way I could use the up/down buttons to set the focus as I like, knowing it won't change. I'll give that a go...