Just to see what happened, I chucked a drag knife in the collet pen holder:
The blade, being fixed in the collet and lashed to the adapter, doesn’t rotate: it’s not behaving as a true drag knife.
Twiddling the cut depth to about 0.2 mm produced a credible Guilloché pattern in some scrap acrylic:
The sidelight comes from an LED flashlight off to the side, with a bit of contrast tweaking to suppress the workbench clutter.
The MPCNC’s lack of rigidity produces visible jitters in the Guilloché pattern and backlash makes the characters somewhat wobbly, but it’s OK for a large and inexpensive CNC gantry machine.
A brace of diamond-point engraving bits are making their way around the planet even as I type. A symmetric 60° point may reduce the swarf thrown out by the drag knife, although it surely won’t improve the overall jitter and backlash.
The Smell of Molten Projects in the Morning 
This right here might make me build an MPCNC. That is really cool.
It’s surprisingly good for a first attempt. Slowing down should help reduce the wobbles in arcs and corner. I tried it at 1 mm depth = 250 g force and that’s just too much; you want the least little scratch on the surface.
I’m tinkering up a real drag knife mount, but I must also figure out how the drag knife compensation works in
dxf2gcode.