If we're going to mention Janus at all, it should be minor things, such as:
1) Calling the school of sorcery the "Janus School."
2) A few shadow spells might have "Janus'" in their name (a commonly used mechanic in D&D, with spells like Bigby's Interposing Hand or Mordenkainen's Sword). A few suggestions, based on Magus' spells:
-Dark Bomb -> Janus' Bomb
-Magic Wall -> Janus' Barrier
-Black Hole -> Janus' Elimination
We probably shouldn't do too many, though, as Magus at the height of his power was less powerful than most average ZEALian sorcerers. What I have above would be alright, as we know that Magus probably did come up with these spells on his own, so Janus could have done so in the ZEAL timeline. They'll probably stack among the lower-level Shadow spells, with new, more powerful spells having been developed since his time.
EDIT: Fixed a typo.