You are correct,...........it is already becoming a big deal as homes get tighter, and especially if icynene insulation keeps getting hotter.
I am in Ocala.
If you like,......I can PM you with my number and e-mail. If you have a digital camera, and could shoot some pics for me, I would be able to recommend something for what you want to do.
Might even arrange for a filter setup at wholesale if one of my suppliers is in your area!
Let me know.
Edit: I think possibly you are misunderstanding what I mean by a "fresh air return." They are required by code for commercial applications, which is mainly what I do now.
In the simple form,.........you would simply run a "duct" to a source of outside air, that ties into your return.
In a perfectly sealed home,.........with a perfectly balanced, and designed A/C system, you should have neither positive, or negative pressure inside your home.
When you turn on anything that creates a "vacuum" you create a negative pressure, or in the case of a non-vented gas fireplace, you run short of combustion air.
With a fresh air makeup, you are allowing your home to become positively pressurized to a slight degree. When you turn on a range hood, or exhaust fan for example,.........the positive pressure is there to negate the outflow, that would otherwise create a negative pressure. It is all based upon the CFM of the exhaust appliances you have in your home, and that is how it is sized.
It also provides needed combustion air for ventless fireplaces, without having to crack windows.
The downside is the re-introduction of humidity into the home. There are many methods to do the same and avoid the negative, but they can become complex for an existing house.
My point is,........you would not be pulling exhaust fumes of any kind into your A/C system with a fresh air return. As long as the intake is not located next to your exhaust fan outlet per se!