Finally got back to this over the weekend and learned a few things. There were several choices of white ink/paint available locally, so the first order of business was to test them for opacity and water resistance:
Dr. Ph. Martin's Pen White- opaque, but no water resistance at all. Ok flow in pen.
Speedball white ink- poor opacity but quite water resistant. Good flow in the pen.
Testor's Aztek airbrush acrylic- opaque and good water resistance, but dries too fast and clogs the pen frequently.
Uniball white ink pen (won't fit the Leroy, but curious about the ink)- Nice line, nicely opaque but zero water resistance.
Daler-Rowney’s white ink- opaque and water resistant, might be the winner.
I think white ink is troublesome because a heavy load of pigment is necessary for opaqueness. If enough pigment is present for good opaqueness, it tends to clog the pen. Thinning the heavier materials hurts opacity. Writing on panels worked well enough with black India ink, but the white inks tend to bleed a bit if they're thin enough to flow well. It's hard to do fine lettering without the letters becoming a blob.
I'm convinced this can be done well, but the right ink is the key. The perfect ink would have just enough opacity, wouldn't tend to bleed and wouldn't clog the pen. To that end, it could take a long time to dry, even requiring a bake to do so. That's an advantage if an error is made, as it can be wiped off and redone. I haven't investigated solvent based materials because of the more troublesome cleanup, but that's another avenue.
More as I try more options.