This is an idea that has also been running through my mind, but a
ducted fan does not have to be driven only by an electric motor. Piston or
even turboshaft power would do just fine. I believe that there are now some
ducted fans in the model world that are almost big enough to do the job.
You would not necessarily need the retractable tail cone, swing out
pressure-operated ducts on each side would also be a possibility. You could
use NACA ducts for the inlet, perhaps also equipped with simple closures.

I found a picture of the Funk Sirius 1 ducted fan motorglider (circa 1970) on
this page:

http://www.fk-lightplanes.com/FK-His...k-history.html

He also built a turbojet powered glider in 1960...

Marc