Not really, no. There's usually a range of +/- 2 skulls on difficulty, but since you're done with the game the game world difficulty is either 4.5 or 5 (I don't remember which) so you'd have to really actively hunt for those uncommon 3-skull missions to grind that rep.
If it's worth it to you as a completionist, then go travel from planet to planet along the Lyran border, either just inside or just outside. Don't take contract travel, because that reduces the number of missions the planet will actually have available to you (odd bug, but true). Eventually you'll find one. When you do, obviously drop the rewards to minimum to max out the rep. You'll only need one or two missions to get to where you can accept 4-skull, and only half a dozen before you can accept 5-skull. It might take a couple hours of real time but you can do it.
