How to fix markup horizontal position relative to edge of page

[Benjamin Bruce]

Hello all,
I am trying to layout some documents where the score is centered on the page, 
and underneath it is some markup text that is aligned left, relative to the 
edge of the page. In some documents the score will be wider, in some it will be 
narrower, but the text below it should always be in the same horizontal 
position relative to the page.

If I set line-width in the \paper block, it will be applied to everything, and 
the markup will be aligned relative to the line-width. If I set line-width in 
the \layout block, the markup is independent, but the score is now 
left-aligned, not centered.

I am attaching my (somewhat) tiny example in case this doesn't make sense. Is 
there a way to accomplish what I'm trying to do, even if it's hacky? Thanks for 
your help!

linewidth_tinyexample.ly
Description: Text Data