Custom chord suffix, no chord name

[Basil Crow]

I'd like to display a chord that says just "Un." -- nothing else. In
Finale, I'd handle this by setting no chord name and then making a
custom chord suffix for "Un."

What's the appropriate way to do this in LilyPond? I read "7.2.3 Printing
chord names," but the chordNameExceptions method seems to just affect
the suffix of an existing chord, which I don't want. I just want to
display a suffix "by itself," so to speak -- although I'm not opposed to
a solution that requires me to use a random (predefined) chord, so long
as the output is the string "Un."

Any ideas?

Thanks,
Basil