occasional ragged-right lines in the interior of mostly non-ragged-right score

[Robert Gaebler]

I am working on a couple of pieces  where I am using a non-ragged right edge 
for the systems.  However I would like to end each of the three or four themes 
in a piece with a ragged right edge on the last system of a theme.
I understand to set ragged-right = ##f to get an even (all aligned) right hand 
edge, and set ragged-right=##t to get a ragged right edge.  However this 
setting affects the entire score.  I would like to make occasional exceptions 
here and there in the score.
Any suggestions?