I would recommend against installing the return air above the ceiling to handle condensation on the ceiling especially when there is insulation installed above the ceiling.
Condensation appears on surfaces when there is moisture in the air and one surface is cold while the other is moist and warm. Condensation is accumulating on the ceiling due to poor air movement and improperly sized equipment to handle the load.
As for indoor air quality, not all batt insulation is rated for use in a ceiling return air plenum.
- By drawing air from the plenum you will be also introducing fibers from the batt insulation causing the filter to clog.
- If there is a moisture problem, you do not want to introduce this moisture into the ductwork which could possibly lead to mold growth.