A few tweaks to the Customizable MPCNC Mount for Round Tools produces a Sakura Micron pen holder:
The pen body seats atop the holder, with its narrower snout inside the clamp, giving positive control of the point position:
Unfortunately, should one forget to zero the pen tip to the paper surface before starting a plot, Bad Things happen to good tips:
The holder really needs at least a few millimeters of compliance, as a fiber-tip pen makes a fairly delicate tool not intended for applying much force at all to anything.
But the holder might make a Z axis probe …
Make a parallelogram living hinge :-)
Exactly like the original pen mount, indeed!