Indeed. You have to 'commit' him before the roll begins. Rerolls are not rolls, they're part of the dice modification step.
Technically, if you have an ability that adds an extra freely rolled die after the main roll (Lightweight Frame springs to mind) you can trigger palpatine at that point, but not on a target lock or lone wolf reroll.
you can 'waste' the result if you roll all natural evades (for example) but I would argue that if you've rolled all paint on what you've identified as the most important roll of the turn, then I think your opponent is entirely justified in dreadsocking you if you start complaining at that point .
I'm not saying Palpatine couldn't potentially do with a points drop, but he is definitely still a powerful crew card, and I definitely think he justifies not being usable on either the TIE shuttle, Firespray or Phantom.