Hello Ben!
The version of fiveballs included in PRS+1.1.3 still saves the highscore. The issue seams to be with the v.2.x.x integration of target.fiveballsRoot with the 600.
I think we shouldn't spent effort on this, until kartu will release his PRS+ v.2 for the 505. Presumably the highscore will not be saved in a separate file, but within the PRS+ save-options functionality.
But anyway congratulations, you are the one and only with a "full-featured, game-enabled" PRS-600.