the telescopic control that mention is not a "telescopic removable control"
he refers to a telescopic mechanism to expand the controller and allow more space between screen and controls
take as example the iPega PG-9055
this gaming controller have two "screws" that allow to expand the grips, for get more comfortable to the user.
think on something similar on the device that allow to expand their structure, for allow to play in comfortable way.
i did some designs based on this, but i discard because are not "practical". i have a mid-state design about this concept now, but i get a little bit busy and did not finished yet, but the main idea is the same, expanding controls (a little bit) for getting better confort on playing, most for people with big hands.
(of course, i know is a design that GPD will not consider really, but i like to create designs)