I have just bought a Spinlock and from studying it, it does unlock with slightly less force than to lock. I think I would prefer to fit it under the bar, because the natural way to twist the bar to release the powerline would be to twist the top of the bar toward you. I am considering mounting it as shown, or using a fairlead cut in half (although I do not think I will find a fairlead big enough). I think this position would reduce the twisting force needed to operate the Spinlock, which seems heavier than I expected. It is the same size as the one you photographed, but with a plastic cleat. I would value your input on this layout.
Ronnie you right it unlocks with less force than to lock,
and correct about the natural way to twist the bar to release.
More Pro riders mentioned to use the Spinlock under the bar the way you're illustrating.
I had couple of rides with the bar upside down Spinlock facing downward.
It was needed some angular adjustment for easy unlocking which is critical and it worked fine.
In fact for setting of my lines and the bar reversed,
downwind of of the kite Spinlock now on top of bar would be better also without getting sand in it.
I found it's a personal preference where you like it.
Myself I like it on the top of the bar in case of emegency,
because at the early testings the Spinlock was needed help with the hand to lock it or unlock it.
I mentioned the angular adjustment is so critical I wanted to see and reach easy from the top.
Once it's set properly it doesn't matter really where it's mounted IMO.
It might also work without the fairled with the Spinlock mounted underneath the bar
and being closer to the rotation center in your case.
Since I used a bicycle stem it needs a small amount of rotation for on or off but needed the fairled.
A big advantage of this system that you can still sheet in for more with locked position.
The layout is fine but you just have to try it in realtime.
Somebody already made similar type of mounting closer to the rotation,
but I haven't got any response how it worked out.
Good luck with it,