I suppose so. I would say it's masterwork until the end of that spell, but it doesn't really matter.
From the Equipment section of the PRD:
"All magic weapons are automatically considered to be of masterwork quality. The enhancement bonus granted by the masterwork quality doesn't stack with the enhancement bonus provided by the weapon's magic."
So, yeah. I would consider it masterwork, but the bonus don't stack and there are no other benefits from it being masterwork.
And then there's this, from the Unarmed Strike section of the Monk class:
"A monk's unarmed strike is treated as both a manufactured weapon and a natural weapon for the purpose of spells and effects that enhance or improve either manufactured weapons or natural weapons."
So, if it's considered a manufactured weapon for the purpose of spells and effects that enhance or improve manufactured weapons, why shouldn't I be allowed to get the Frost special ability (which I have now with my brass knuckles) or any other special ability? The unarmed strike is considered a manufactured weapon, just like your dagger, a longsword, or a Dwarven urgosh. The same rule should apply.
This is the argument I used with my DM in order to get rid of what I thought was kind of a unique, but ultimately, annoying weapon, the brass knuckles. Our game is laid back. We're a 'beer and pretzels' kind of group. As long as it's fun and I'm not 'breaking the rules' (which nobody thinks I am), then everything's fine.