The air intake is usually along the bottom of the chin, and on some models - there is another intake behind the foot stand.
Someone had a excellent point earlier up - iMacs sit on desks - which generally are orders of magnitude cleaner, as they're away from the floor. They won't be sucking up pounds of dog / cat / human hair, along with coarse dirt like a computer sitting on, or just above the floor.
As for the filter idea: That thin layer of dust is not NEARLY as bad as putting a filter across your air intakes. You've got a couple of things to consider. Is your filter media anti-static? As you're going to place it directly next to one of the most static sensitive components in a computer - your ram. (unless you plan on mounting it externally) You will have to seal the filter along the edges - or else the air will blow by anywhere you don't have a hard seal, or at edges and corners. Also - even a "high flow" filter impedes airflow a LOT. So if you do achieve a good seal, your fans will be running faster in order to maintain the needed rate of airflow. If you have any other intake ports that you don't affix a filter to - the pressure delta will draw even more air through the uncovered ports.
Finally - that really fine powdery dust - guess what? Unless you're cutting pleats of HEPA or ULPA filter media - it won't stop that dust at all!
That said - your computer, your endeavor - I wish you luck, and if you have success - please share it! (I'm still interested to see)