Foobar2000 can do this. Add the playlist, Ctrl-A to select all, right-click and choose Sort-> Reverse. However, Foobar can't output the PLS format, so if the Squeezebox can't handle M3U you would need a way to square that circle. Foobar's default behavior for M3U is to save in relative format, although there's probably a way to change that.
Incidentally, my main playlist is about 8,500 items. Adding it to Foobar takes a moment, but reversing it is basically instantaneous.