Technically, within the armoury system, the weapon type is (for example) not "axe" but "marauder's arm".

As a purely visual feature, I could see this being totally fine. As long as the weapon type doesn't require any extra animation work, why not?

I think all of the current WAR animations would work just as well with a hammer (the Allagan battleaxe is so blunt it practically already is one).