How difficult would it be to install one of these on an ES once the wing has been closed? 

Mark,

 

I don't know the ES wingtip, but there are always ways...........

 

I think other posts have suggested some methods.  I did mine after the wing was painted and the only touch-up is to the bottom where the drain tube was installed.  I did go thru the position light and then cut a hole in the wingtip rib.  I use the professional version with a pleasant female voice nagging "Angle, angle, push!" at critical times.  I have not tried to approach an inverted stall where one might expect "Angle, angle, pull!" (Hmmmm.....)

 

I have other nagging type female voices in my headset - such as "Traffic Nearby!" or "Stop and pick up some fresh fish for dinner!" or "Did you forget our anniversary again?"- each requiring immediate attention.  Well, some are life and death and some are just death if ignored, you pick those that are important to you (like "Beep, beep" and a gear warning light).