Just to add as a follow up, as I've been continuing hitting my head against this problem...
I tried connecting the power meter on a different PC, via tmtool, and it worked successfully. Copying the line that tmtool generated to make the connection, I noticed that it was using a different vendor name than my own code:
PM100A = visa('KEYSIGHT','USB0::0x1313::0x8079::P1002477::INSTR');
I tried executing that line of code on one of the non-working PCs and get the error:
Error using visa (line 224)
Unable to create VISA object. The required drivers may not be installed properly.
So, then I tried uninstalling and re-installing the meter's drivers in Windows Device Manager, doing likewise for the Instrument Control Toolbox in Matlab and...same error.
That's me totally out of ideas now. It's not obvious at all why it works on some PCs and not on others. They all seem to have similar/same Matlab versions (2019a or 2019b), and the same toolboxes installed. All have the same drivers installed from the same vendor, all work fine with the Thor Labs software. It's a total mystery to me!