pinch to zoom (the default one) might be a problem if you want a HUD layer.
Perhaps, on two touches at the same time (not sure you can even do that)
create an invisible object at each one
set layerscale(whatever)
clamp(factor / distance(x1,y1,x2,y2), lower zoom out limit, upper zoom out limit
factor can be anything... not sure of the math now but play around with some numbers.
Hmm, this resets it every time you try to zoom though...