On this weird sideshow to the assassination, I have to side with Wild Bill. It just seems like something Booth would do. That, plus the fact that Herold was working at a place that supplied drugs and likely other things to the Lincoln White House, seem to tie it in to Booth's band.
I recall thinking when I read about the window inscription in Weichmann's book, "Well, that seems plausible, given Booth's bragging after shooting Lincoln."
Can it ever be proven that Booth scratched those words? Probably not.