Marc, I've been presuming that the rule covers things like sword-breakers and trident daggers/sword-catchers, as well as special techniques. I might be entirely wrong though!
Looking at the rule as it's written, I think it already assumes a common-sense/sculptural-advantage approach. Say you have two beastmen, one with natural weapons (horns, claws, a mace tail, whatever) and the other with an axe. Their profiles are identical, but the first one can't be disarmed and the second one can - simply because of the way the models are sculpted. I think that's fine - and I see a model having to spend an action to draw a sword instead of potentially spending two and risking a free hack to retrieve his main weapon as a similar thing.
I had a quick glance at the miniatures I've been painting up in recent months, and the great majority have only a main weapon. The exceptions are mainly historical figures - knights and so on (I bought a box of the fantastic Perry foot knights some months ago). Interestingly enough (to me at least
), the historical figures tend to be armed with a "trait" weapon and a normal one. So, many of the knights have poleaxes or lucerne hammers (both Piercing Weapons) as well as swords. Similarly, a lot of medieval footsoldiers are depicted with a spear or bill (Long Reach!) as well as a sword.
Given that, and given the rarity of the likely occurrence in the game, I think a "use common sense based on the miniature" is fine. I think it will also create interesting tactical choices. If your knight has just been disarmed, do you risk a free hack to go for his Piercing/Heavy Weapon, or does he just fight on with his sword (forfeiting an action and with a reduced chance of a kill thereafter as a result). In certain circumstances, the free hack might be the only hope of success.