Yes, it was a stupid omission on that amplifier. The loudness switch should have been a mono switch IMO.
You can do it. You can switch the rear pre/main coupling to separate, put two coupling links from in to out and then place a switch between the two 'hot' links. It would be a mono/stereo switch. No modifications and could be extended out to the shelf the amp sits on. Easy.
I would use two core+shield (balanced mic) cable and a small toggle, soldered and heatshrunk on about 40cm of cable.