Non-justified plainchant (Gregorian and modern notation)

[Benjamin D. Smedberg]

I am new to Lilypond, having been a user of Finale for many years. I am 
a relatively experienced programmer, having extensive experience within 
the Mozilla project. I have several questions, but I'll start out with 
something I hope is fairly simple:

I do quite of bit of engraving in plainchant, both modern and Gregorian. 
I have not discovered a layout mode which does not justify the music on 
each line. In plainchant, the staff lines should extend to the end of 
each line, but the actual notes and text should be spaced and engraved 
as if it were left-justified (except for the custos). Does this mode 
already exist, and I missed it? Or is it something that should exist as 
a scheme extension, and can I get guidance on how I could code that?

--Benjamin Smedberg
Director of Music, St. Patrick Church: Washington, DC