These cameras can't do continuous focus (there's been some advanced talk about custom focus algorithms and the like before, though), so this would have to be something along the lines bongo_bingo says...
I.e. keep focusing until you full press or try to be smart and figure out when there's a need to focus again.
MD is one way to try the latter, but keep in mind that it's never been done outside scripts (well neither has the continuous focus thing, but that would probably be easier to hack) and it's a beast to configure in a thing like this. If such a thing would actually work, maybe a simpler detection from histogram content change could work...?
But a good compromise would be to assign a key to force refocus during half press (the obvious advantage here to "release shutter -> half press shutter" is that this is way faster due to autoexposure being skipped, among other things). This should be pretty easy to do if we know how to force refocus during half press.
