nothing easier than that, the original stand is alloy or? so just go to the next workshop who can weld alloy and get a piece welded in so you get the right hight and one extending the foot a bit more , if they do a good job and get the proportions right it will not tumble over due to the weight shift and you will never guess that it was not original like that
I sure wish they'd put a vesa mount on the new 21.5 coming my way soon. I have a 22" monitor mounted to the wall right over the top of my desk. I love it. That being said, the pedestal under my 17" imac is plenty high off my desk for my tastes. I just like the convenience of being able to position the monitor off an arm.
yes i have posted this vesa mount adapter in another post here on the forum , but if you only want to rais the hight it still leaves the stand hanging in the air than ok maybe you can utilize the stand then as a shelf for the keyboard
and yes you can turn it even 360 degree if you fit a arm to the vesa mount
This is the VESA bracket I use for my 22" extension monitor. It has a 33 lb capacity and that should be plenty for a 17, 20 or 21.5 imac. For 20 bucks, ti's a good arm. If you don't mind drilling out 4 holes in your pedestal, it's a 20 dollar total solution. If you're adverse to that idea, the imacmount thing obviously works, but at a price.