I wonder if these might help:
http://blackisbeautiful.ca/portfolios/tamiya-x-1-paint-marker/Tamiya has this in white bought a few years ago for white trim on walkways for locomotives. Worked well. I think some automotive touch up paint pens work on the same principle. You can alter the chisel tip with a sharp blade to get the size you need.
Also GSI Creos makes a line of markers for the Japanese Anima and Science Fiction Fantasy model kits (I don't judge...). I'm sure white is available. For example:
http://www.amazon.ca/GSI-Creos-Gundam-Marker-Markers/dp/B0009AFN0SAvailability of this stuff can be hit-or-miss. I know my local hobby shop has them. Seems like stuff available here in Canada or the ROW (Rest of the World) may not be available in the US for whatever reason. Kinda like Outers GunSlick (anhydrous graphite) that I used for lubricating locomotives. I can get that when in the US but not in Canada.
Hope this helps
