Alright, so I have a giant telecentric lens on my machine at the moment (and a 150 on deck I can switch to... and two other focal lengths on the way) and used Kev's settings above (600mm/s, 99%, 0.006 cross hatch) and you're right, it was really quite interesting how just 1kHz made a difference. I ran 2mm squares on polished (almost mirror like) brass tags from 80 to 30 kHz at 1kHz apart and found:

60-80kHz: Not much of anything. Very faint and inconsistent marking
mid 50's kHz: some nasty, patchy, tar black on the first pass that mostly got cleaned up when the cross hatch came through
43 kHz: Actually a pretty good frosted/matte white
30-39kHz: Tans, with 32kHz being the darkest tan

I've got a bit more poking and prodding to do-- but this is fantastic progress and I appreciate everyone's help!