Custom clef

[address@hidden]

Hi list,

I have a cello score using 2 staves, the extra being used to display movements 
of the bow (sul tasto / sul ponticello), using glissandi (on a staff with a 
single line).
This extra staff has a special "clef" that looks like this:

SP
ST

How can I make this text behave like a clef, i.e. repeating at the beginning of 
every line ?
I searched how to define a new clef or how to make a \markup block behave like 
a clef, unsuccessfully.

Thanks for your help! 
Martin