I don't see a problem using a rotary table for either aluminum or steel. I'm not sure what tire width is needed for a 1 1/4 blade, but I assume the wheels are going to have spokes with a lot of material removed between. If using steel, you will be starting out with a lot of weight so take that into consideration, about 111 lbs for 20'' dia x 1.25 thick. In that case I might be tempted to center the work on the machine and rotate the ram off center. Again I am assuming that you are using a BP size knee mill or larger. I have made some pretty large disks on a RT. Adding some kind of power feed to the RT would save a lot of cranking.
I might start out by having the wheel roughed out with a water jet or plasma cutter. Water jet would be best if you're going to use steel, the plasma would leave a hard edge on the cuts.
Another crazy thought comes to mind. You could build a ''lathe''. Do all of the work on the center first and get the bearings in place. The mount the wheel up, provide a way to power it, and mount up the compound off of your lathe to turn the OD.