While not the most elegant sounding solution, I've used wire, cable, chain etc. to mount other non VESA type televisions and such to a wall.
If the iMacs stand can withhold the weight of the iMac, I'd probably take some heavy cable and wrap it under the hing point where the foot meets the back of the case, and then make a loop of cable above that point and hang the Mac from the cable and a good solid bolt in the wall (bolt would need to be in a stud in the wall).
Adjust cable until you have a good viewing angle. If you put some planning into it, you can conceal the bolt and the cable behind the imac's case / screen.
That would leave the foot in place. Use plastic coated cable if you want to prevent scuffing the foot.
I know it doesn't sound elegant. But I've hung some flat screen televisions that way without trouble that didn't have a wall mount option.
It's worked fine. Just make sure the cable and bolt are strong enough for the weight. And test holding the iMac in your hands at that anchor point to see if it feels sufficiently solid there.
Naturally, I make no personal assurances. The task is up to you to judge suitability and safety of the option. In other words, try at your own risk. Just stating things I have successfully done.
