How does one make a skill like the weapon skills, which require a specific type of item be equipped (or not equipped) for the granted bonuses to take effect? I've pored over skillDB but can't seem to locate anything which could be what controls it.
It's hard-coded right now. If you want to make a skill tree for a whole new class of items, then you can circumvent that somewhat, but it's coding hell with a whole lot of copy-pasted dummy triggers (and in this particular case I speak from experience). Then again, if what you want is simple, then giving all items of the new class an on-hit brittle dummy buff that lasts a few hits and making all bonuses and abilities from the new skill tree require said buff will suffice. Coding hell is only there because what I'm doing lies near insanity.
David has hinted that they might be adding XML hooks so that skills can require/trigger off of specific items in the near future. How near, we don't know, but it's definitely on their mind.
And thus there will come salvation for most people. Not for me, obviously, since what I'm doing is borderline insane, but you guys seem to be luckier than me (I blame that on my sleep deficit).