If you strip the iMac down to the bare chassis and sand it with a suitably fine sandpaper, you could get a decent result. Without testing it, it's difficult to know how fine, but something in the 1000 to 1500 grit range might do it. You'd have to be careful to keep the sanding motion all in one direction and you could get a decent brushed aluminium finish.
The small snag is that the Apple logo is painted on, so I think to do this and to keep the brushed aluminium effect looking decent, would mean losing the Apple logo.
All up, it's only something I would try as a last resort, i.e. if it all goes horribly wrong, you can always buy a new chassis afterwards.