Developments in the ascender/descender/work positioning mechanical device market are coming in thick and fast - the latest from Petzl is shown below.
The RIG is a self-braking descender (or short ascender) with a multi-functional handle which allows comfortable control of descent and you can position yourself while on the rope without tying off.The handle returns automatically so you don't set the RIG in action by mistake. The handle also has a storage position which reduces the risk of snagging when the descender is being carried on your harness.
The RIG is pictured being applied in a top-down (from anchor point) application but with a little imagination your could see it being used successfully in tree work.
The RIGs ability to make short ascents and to take up slack with very little friction on the device itself would be an added bones. We have not held the RIG in our hand but it looks a little bulky in the video. ie larger than a Gri-Gri or Grillon.
Size certainly counts - that's why the Trango Cinch and the ART Positioner are so popular as work positioning lanyard adjusters.