Ask and ye shall receive...
I trimmed off the plastic standoffs that held the slightly narrower 28 Ah OEM battery in place, and the pair of 12 Ah ones fit in perfectly, not a millimeter to spare!
That makes me wonder if the mower may have been originally designed to possibly take two 12 Ah batteries.
Haven't fired it up yet, though. Next I need to make up some wire connectors to hook everything together.