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.