Guitar Barre'

[Maurizio Monge]

Hi, i am trying to put a barre' in my score, it should be something like:

C. III_______________________________,

above the score, and the only way i was able to do something like that
was overriding the 8va to write "C. III" instead of "8va":
            #(set-octavation 1)
            \set Staff.ottavation = #"C. III"
            ...
            #(set-octavation 0)
Is there a more intelligent way to do this? 
I searched in the manual but found none. 
Without considering that this way the midi file is messed up :-)

Thanx 
Maurizio