Thanks for sharing your thoughts, guys! It's much appreciated.
The exploded diagram from toolservicenet was certainly helpful. The backing plate pattern doesn't appear to conform to any specific standard, though it looks like it may be potentially shared across multiple brands. I did also come across another that may fit, which is intended for another brand called Craftsman. After a bit of Googling around, it seems they both share the same parent company.
There appears to be a number of pads from different manufacturers out there that may fit but they all seem to be 5 inch pads :-(
Perhaps the positioning of the four mounting screws prevent affixing pads of any smaller diameters. I didn't manage to find any smaller options or adapter plates that would work instead.