I haven't, though I've seen the post. This wouldn't help me layout in the designer anyway, which is what I really want to do.
If there is no alternative, then I'll have to. It just feels like this can't be the correct answer as it make the designer unusable.
I've found the setting android:fitsSystemWindows but setting/unsetting just seems to make the navigation bar translucent or not but leaves the button behind the navigation bar.
I've tried just positioning it at the bottom in the designer and anchoring it to the bottom, and also setting Button1.bottom = 100%y in designer script.