Right- and left-aligned LilyPond page numbers

Use \fill-line with an empty " " string followed by the magic page number incantation to right-align page numbers.

   \markup {
      \fill-line {
        " "
        \on-the-fly #print-page-number-check-first
        \fromproperty #'page:page-number-string
      }
   }

Swap both arguments to left-align.

   \markup {
      \fill-line {
        \on-the-fly #print-page-number-check-first
        \fromproperty #'page:page-number-string
        " "
      }
   }