How does Reprisal work with delayed mechanics like meteors?

Just as a PUG-lifer Tank in PF for my first on-content raid tier I got to learn that using Reprisal before raid-wides or tankbusters (if the other tank is taking care of raidwides so we don't accidentally stack waste them) is good practice.

But how does it work with delayed mechanics like meteors?
Like Proto-Ozma's (or Ozma)'s meteor marker proximity mechs? Or E4S's placement meteors in phase 1?

Does the Reprisal reduction take effect during the cast bar? Or should it be active during when the meteors get "placed" and lock in where someone puts it?

At some point, the outgoing mechanic's damage will be "snapshotted". Reprisal needs to have finished its animation and be applied to the mob using the mechanic before this snapshot point.

Where this point is varies from mechanic to mechanic. Sometimes it's before the cast bar is over but as you've noted, some mechanics are delayed significantly beyond this so it's not a reliable anchor. You could also say something like "3 seconds before the damage applies" but that's also not always reliable.

The only real way to know for certain is to experiment on a per-mechanic basis. For high end content, guides will often note if a mechanic needs special reprisal/addle timing. If there's no special notes in the guides, just do "before the cast bar ends" or "a few seconds before damage applies" and you're probably OK.