To be honest, I don't know an elegant way to do this and I would be very happy to see one. However, in the meantime, you can engrave it with use of a hack like this: lilybin.
What I did there is essentially this: I attach a markup to the last note in the right hand like so:
b-\tweak extra-offset #'(6 . 1.5) _\markup{\with-dimensions #'(0 . 0) #'(0 . 0) \center-column{ "D. S." "al" "Fin" }} \bar "S-S"
The main idea is to take what you want to typeset, which is \center-column{"D. S." "al" "Fin"}
, make it take up no space using \with-dimensions #'(0 . 0) #'(0 . 0) ...
and put the markup to the right place (found by trial and error) using -\tweak extra-offset #'(X . Y)
. It's very ugly, but sadly I don't know of anything better and it at least gets the job done.