I'm not widely known as being a "softy" of a GM. But I might be willing to let a character spend a feat on something that they don't qualify for with the understanding that they can't use it until they DO qualify for it based on something like BAB.
For example, if somebody wanted their 1st level Rogue to take Weapon Finesse, I might allow that even though they don't meet the +1 BAB requirement. They would of course gain no benefit from it until they reached 2nd level and got the required +1 BAB, but it would let them take the feat earlier than 3rd level. To me getting the feat 1 level sooner than they otherwise would have is balanced by the fact that they essentially lose access to the feat for the duration of 1st level.
Part of the reason I'm ok with this is that the game supports the concept of characters having feats they can't use at present. If a Fighter gets his Str drained below 13 by a Shadow, he loses Power Attack, Cleave, Great Cleave, the whole chain of feats that were predicated on him having a 13 Str. But when he again qualifies for the feats (i.e. when he gets a Restoration on his Str), he gets them back.
I say all of that with the caveat that I'd keep a close eye on how players were using this house rule and I'd be quick to shut it down if it seemed like it was getting abusive. Fortunately, that's not something my players tend to do and actually, to date, nobody has tried doing this anyway. I think most of them (correctly) believe that having that feat NOW when it might save their life is more beneficial than some potential future benefit.