alternate bar numbering

[james]

I haven't seen this documented, or if I have, I've forgotten where, but is it 
possible to manually set an alternate bar number, i.e., with a letter? 
\set Score.alternativeNumberingStyle = #'numbers-with-letters
\set Score.currentBarNumber = #10a
doesn't work, and I didn't see anything that explained if it's possible or if 
so, what syntax I should use.

Thanks for any help,

James