Thanks for all the suggestions folks. The jury is still out on what
method to use as I have no experience at all with pyrography (pens) or
whatever. My handwriting isn't that good at the best of times, and I
suspect that using an unwieldy writing instrument as well would make the
end result worse, thus somewhat negating the point of marking my work at
all! The idea of using a very fine marker pen and finishing on top of it
appeals, but I wonder whether the finish (depending on what is used)
might make the writing run.

--
Alun