I assume (oh dear...) that we are referring to the winglet being canted in (in relative position to the longitudinal axis) towards the fuselage, whereby, because of flow patterns and deflection, it more or less augments the airflow in the area, with the fuselage acting as the opposing augmentation area - kind of like the turkey feather nozzles on afterburning engines. A funnel if you will, or even perhaps a venturi?
If so, yes, there WOULD be a net thrust generated, but how negligable it surely must be! It has to extract energy from the airstream to provide a LIMITED amount of thrust, perhaps at a small increase in FBO because of an increase in parasite / induced drag caused by the winglet.
In reality, I would consider it a "six of one, half a dozen of the other" scenario. The benefits and the negatives surely balance out!