After searching the 'net for hours, I've found that there's a 2.75" x 21" tire. Sad part is that it's only available in offroad, trails, enduro or dual-purpose. The last choice looks like knobby mountain bike tires, but for motorcycles, like this one:
Using reference information, rear tire side clearances on my rear dropout should increase by 1/4". The tightest left-side chainstay clearance would be ~1/8".
Looking better, might not need to clearance the rear dropout with a 2.75" x 21" m/c tire.