I found an obscure (and old) reference on the Wizard's BBS forums,
which basically said that they intentionally left this out, but didn't
In 4e, rules are always designed to benefit PCs first. A disarm mechanic would be disproportionately good for NPCs, as most PCs get great benefit from weapons and implements, while many monsters don't use them at all. Hence, no disarm mechanic.
For realism, a disarm mechanic would be great--but 4e is pretty explicit about its lack of concern re: realism.
In answer to the main question, I would probably ad-hoc something like an attack roll against the better of the target's AC or Reflex, with the target adding their weapon proficiency to their defense (and possibly a bonus to defense for two-handed weapons as well). Penalties for being disarmed would be straightforward: use unarmed attacks for weapon keyword powers, and lose all weapon/implement benefits until the item is picked up again.
The problem with any freely-available disarm ability is that it seems almost universally overpowered for NPCs/monsters, and only situationally useful for PCs, which as stated above goes against 4e design philosophy and might be unbalancing. Anything your players have universal access to, their enemies probably do too. I've found this is an effective way to get my players to back off on some of their unbalancing requests: "Do you really want Davith the Wicked to be able to do that to you?"