Rehearsal Marks

[Jennifer Clark]

My apologies for yet another question, I have tried to resolve this myself but 
cannot find the answer. Here's the situation - I can create rehearsal marks:-

With letters, boxed, by manually adding text
With letters, unboxed, automatically
With numbers, unboxed, automatically
With numbers, boxed, automatically (example in the manual)

What I'm looking for is letters, boxed, automatically. Is this possible?

BTW, I tried out the the charts I prepared with lilypond last week with our 
jazz combo last night, and received no negative comments whatsoever about the 
layout - praise indeed, believe me. So congratulations on a good program!

Thanks again,
Jennifer