I have the fortune in this area of working at a steel mill. I found some 1" angle iron laying around. I cut four 2" pieces and put a thin slot at one end in one plane of the iron so that it fits over the outer ring of the turkey fryer. The keg rests on these and the weight of the keg lodges the pieces in place (that and the tight fit by using a thinly cut slot). Probly not much use to you since not many would have this material available.
As far as your solution, I would be concerned that since the grate goes right over the flame, this could get red hot and lose its strength allowing the grate to deform and possibly in an uneven pattern and your keg could tilt.
Edit... now that I think about it, get some 1" diameter by 2" long pipe nipples or couplers from any hardware store and cut a thin slot in them about a half inch away from one end. I would only cut the slot about a third of the way through so it will maintain its strength. Use a dremel with one of those cutting wheels and this makes a pretty good slot width. Four of these around the perimeter will increase the effective diameter of the burner by 3". The outer ring on my burner has proven plenty strong enough to hold a full 15gal keg. I wish I had a digital camera sometimes...