War Clerics (and other PCs) are capable of it through the Bastions rules in the DMG.If masteries represent skill-at-arms beyond Proficiency, then war clerics as-is either:
a) aren't capable of that kind of skill with weapons - in which case, why not?
or
b) their weapon skills are limited (by their god, I assume?) to only whatever buffs their god (i.e. war domain class features) grants them for weapon attacks.
It just seems weird to me.
It's just a more time invested process for the PC than a class feature. An interesting impact is there's a time limit on how long the benefit lasts so that creates added incentive against the 5mwd.
It's also an opportunity cost against other bastion options.