It is hard to say if what you are doing is ok. We need to hear more details about how you intend to construct this duct system. Do you intend on just enclosing the three sides of the 18" trusses with ductboard, cut out your openings for the registers, and direct all the air back to the furnace. If this is the case, and it is all that you have specified so far, then no it won't work. Because the air ducts need to be sized properly to get the correct air volume and velocity to each room. This is calculated by heat loss and heat gain factors. I have worked on condos that didn't use an enclosed duct system and even on a small scale the performance of the system was very poor. Of course now that it's done to change it to a ducted system would be very expensive. So the tennants have to deal with low levels of comfort, mostly in the A/C season. If you do not size the ducts or enclosurer the system will not even come close to working right. So now that you know the ducts need to be sized, is it worth your time to mess around cutting and securing ductboard to the plywood? I have installed many systems and I just don't see how what you intend on doing could work and/or be worth the time.