This isn't quite a good as a partial disassembly but it works surprisingly well.
Run TechTool Pro 8 (might work in 7, not sure) and do the default tests. One of them checks the fans and, in so doing, ramps them up to full speed. I would strongly recommend you do this outdoors as, if your iMac is full of dust, it will come shooting out the rear slot in an enormous cloud of filth that will cover everything in the room. I made this mistake once but never again.
After I ran the test, powered down, and opened up the iMac to swap out the HD for an SSD, I saw that most of the internal dirt was, indeed, no longer there (but on my furniture). I did use some brushes to get the remaining dust stuck to the fan blades removed and vacuumed out the interior; but the TechTool Pro test was a good stopgap if you're not in a position to open up the iMac "right now". Just remember to do it outside!