Since you're only allowed one, you can enable cat codes OR try the WB EGR thing.
GM cat was triangular in shape IIRC, so that might help in identification, but that doesnt matter. It only matters if it will pass the P0420.
Since the EGR isn't there, if you want it to show "ready" you'll have to hook a wideband up and hook the WB output to the EGR position wire. Having done that, my PCM is convinced my EGR is still operational even with all of the codes disabled.