How does one add an "x" or some other mark within the staff where a note normally goes?

[Ben Beeson]

Hi,

I am looking at a handwritten score that contains some extra marks in the staff that I would like to include in a LilyPond score. In the bottom part of the score linked here

http://www.altpibroch.com/PrimarySources/185-K1_5-Lament_for_Catherine.pdf

you can see where the author as written in some "x's" in the staff at the C or A note location. What I would like to do is replicate that marking in the LilyPond score. I have tried markup, but that does not place the "x" in the right spot. Is there a way to make a note have zero time value and then replace the note with a letter or other symbol?

Thanks in advance for your help,

Ben