Probably not frescobaldi,musescore,mup*
*
http://www.arkkra.com/doc/mupfeat.html
Best would be of course if Reaper would allow mup text file export! Not for editing but for printing, creating pdf files.
I am thinking of following workflow, after each switching from midi editor to arrange, you would trigger some custom action which would export to mup text file, convert to pdf, display updated pdf, on second screen for example. So you do your normal editing, but in parallel you get automatically all score update on second screen, in best possible musical form, kicking away anything else like Finale or Sibelius or lilypond.