ORIGINAL: Torqued
Ahh, so they already to have them
no surprise there! My design thought is somewhat different though. I envisioned the duct to have only a drive shaft with the engine outside the duct. It would seem to me that the engine would greatly disrupt the flow of air from the fan in the designs in the above links.
Here is more of what I was thinking and PLEASE excuse the extremely crude drawing
Ducted fan engines usually turn over 20,000 rpm so a drive shaft may have to be heavy and complex (braces and bearings). Also there would be no engine cooling unless you were moving forward. I think the outlet should not be a nozzle also. There are my non expert thoughts.