I fear there's no way I'll be able to obtain the Final Judgment or Maxson's robes, so I might as well save someone else the trouble.

*Railroad Related Spoilers Inside*

If you side with the Railroad and start the quest Rocket's Red Glare, be careful about how you handle the Prydwen. When you arrive there, you will have to get to the main deck to handle some business, and then return to the flight deck to depart. You can do this through conflict or stealth. If you make the mistake I made however, and choose stealth, Maxson will never spawn and you will lose your final opportunity to acquire his unique robes and gatling laser.

Based on what I have read from users with the same experience, Maxson is supposed to automatically spawn in his command room during your escape with two other knights who will immediately light you up like a Christmas tree, and in this hectic moment you are able to kill and loot him for his gear. However if you entered the Prydwen and passed through the command deck undetected before handling that business on the main deck, this event will never happen and you will just be queued to leave. I can say from experience that turning around and killing everyone you snuck past won't matter at this point.

I'm quite bummed, and the game appears to have started only creating 3 autosaves at a time, just enough to ensure that the only way I can resolve this is by loading an 8 hour old hard save. Don't be a victim, go in there raising hell so you get your loot.