B132L 10.20 user-defined display resolution

[benofiniquity]

I have a batch of B132L workstations running 10.20B.  I am in the process of upgrading them, but it's slow going due to the cost of the proprietary software (~$400k/machine).  In the meantime, I figured I'd at least throw them all in a rack with a KVM to minimize the footprint of my albatross.  I ended up with a 1U console that will do 1280x1024@60Hz max.  Unfortunately, all the provided resolutions are 75Hz, save for 640x480, which isn't a useful resolution.

The boot-up display selection lists 15 resolution/refresh options, with the last one being " -- user defined --".  Anyone know how to define it?  I've poked around in the boot menu, but there don't seem to be any options related to it.  /opt/graphics/common/bin/setmon seems likely, as it has command line options to erase and write ROM monitor settings, but I'm getting "hardware not supported" errors.

So, I'm out of ideas.  These machines are old enough that it's hard to find deep documentation outside the user and service manuals.  I've got no qualms about dumping firmware and editing by hand (I have a few spares to play with), but that seems drastic for something that should be straightforward.  Any clues or avenues to explore?

Thanks in advance.