Could you add the permission essentials.seen.exempt ?
I think you understand its utility

But, if it's possible, add possibility to /seen someone who have this permission if we are OP.

If a player have the permission essentials.seen.exempt, an other player with permission essentials.seen can't /seen a player with permission essentials.seen.exempt.

This may not work consistently with some permissions plugins as the player will be offline, but it's a valid feature request.

If someone comes across it and still feels like making a PR it's on the table. It just seems there is not much interest in this issue, so I will be closing it.

Edit: And also as md mentioned above it might not be a very stable addition to the plugin. Be warned.