Does anything preclude a Paladin from using his Divine Bond ability on a weapon he conjured — e.g., using the Bard spell Instant Weapon (by way of the Unsanctioned Knowledge feat)?
There is nothing preventing it from working, but it is probably not a good idea. When a paladin’s bonded weapon is destroyed they lose the ability for 30 days and during that time take a -1 penalty on attack and weapon damage rolls. Considering both bonded weapon and the spell have a duration of 1 minute per level that means that this is going to be triggered every time. Paladins caster level is equal to his paladin level -3. Since you cannot enhance something that does not exist you need to cast the spell first. I can see a paladin using this as a move of desperation when there is no alternative, but not something they would normally do.
You bring up a good point. I didn’t think of the spell expiring as being the equivalent of the weapon being destroyed... but I can certainly see how that would be the logical conclusion.