After shimming the corner posts, a plot with Sakura Micron pens came out nicely:

They’re all 01 size pens, with a nominal 0.25 mm line.
Just for fun, a plot done with four sizes of black Sakura pens at Z=-1.0 before the Great Leveling:

The 005 pen made a nearly rectangular single-pass tour around the perimeter of the plot, so you’ll see it passing through every legend.
The chunky-by-comparison 08 pen = 0.50 mm:

The 05 pen = 0.45 mm looks much crisper:

The 01 pen = 0.25 mm:

The almost-can’t-see-it 005 pen = 0.20 mm:

If you were doing this for a living, you’d probably use 05 pens, because plotter pens are hard to find.
Original HP plotter pens produced a 0.3 mm trace (with a hard to find un-worn tip) roughly equal to Sakura 03 pens, but I haven’t seen anything other than black at Amazon. There’s apparently a 003 pen with a 0.15 mm line; that’s just crazy talk.
Jamming Sakura pens into a plotter pen adapter for the MPCNC makes little sense, so I should gimmick up a specialized holder with some thumbscrew action to keep them from crawling upward out of the holder.
From experience, an 005 pen tip will wear away pretty quickly under this duty (I’m using a Cameo Silhouette with an art pen collet). The fiber point will wear down to the level of the metal sleeve it pokes through. Being cheap/frugal, I file new points on worn out drawing pens anyway. You don’t get the same size line anymore, but you can at least use it for writing. I find the Sakuras wear out before they run out. Anyway, when they do I just sharpen them with a diamond file like they were a pencil.
If you want to lay down a lot of ink, check out the “Graphic 1” size. Graphic 2 and 3 are chisel points and may not be what you want at all. I tend to use 005, 08, and Graphic 1 for drawing and 01 for writing. The 05 holds up well in the Cameo.
I tried pulling the tip further out of the sleeve, but they’re firmly anchored inside. Using a saw on the sleeve seemed too dramatic; I never thought of a file. I’ll try rehabilitating a couple of wrecked pens and report back.
Thanks for the experience & suggestions!