Modders have successfully unlinked Uplay requirements by changing the weapon values inside the game's core data files.
If you have unlocked it in the past, playing in "Offline Mode" within the Ubisoft client will usually allow you to keep using it. Far Cry 4 A.j.m. 9 Unlock Without Uplay
If you prefer a plug-and-play solution that alters the in-game storefront rules directly, community mods are highly efficient. Popular total-overhaul mods modify the game's internal storefront XML parameters so that Uplay weapons unlock naturally alongside standard gear. A.J.M. 9/FC4 - Far Cry Wiki Far Cry 4 A.j.m. 9 Unlock Without Uplay