Good news! You already can have access to Uriel Blade!!! All you have to do is farm up the required Allied Notes, purchase Griffinclaw, stick it in your main hand, and get to work in some Campaign Battles.
I'm sure you want a way to quest it for permanent use on every job, but I'd much rather have access to quested Relic skills for every job before Campaign WS. Then again, those should be able to be quested to any weapon as well.
Also, there are more Campaign Battle NPC exclusive Weapon Skills that players can't even access at all. It would be an interesting new thing using old assets (WS dat files already exist) and that is what SquareEnix loves to do. Honestly, they could (and probably will eventually) do this down the road.
Look forward to it being unlocked through a ***-ton of those 40 monthly Deeds of Heroism. Actually probably what the 960 Deeds reward is. The "Treatise on Martial Techniques". Actually, how do they know it's supposed to increase your merit points you can spend in that merit category? Is that by mining the dats or just a guess?
https://www.bg-wiki.com/bg/Treatise_on_Martial_Techniques
I think I'm onto something. Time will tell. Wonder how many more months before people that have done 40 deeds worth each month since inception will be able to find out.