boxed rehearsal mark "I"

[Jeroen Gijzen]

Hi,

I've got this danish piece of music which uses the letters I, Æ, Ø, Å, 
A2 and so on for rehearsal marks. When I use

\mark "I"

I get a mark without a box. Is there a way to use an arbitrary character 
with the \mark command? If not, does anyone know a workaround for this 
problem? I'm using LilyPond-2.4.5 on RH9

Thanks,
Jeroen