Non-PF monks get Imp Trip as their 6th level bonus feat, but the OP stated he's 6th level, and has at least 1 level of barb, so that ruled that out.

Also, you don't need a house rule for INA to work for a monk. UASs are natural attacks in ALL regards, EXCEPT for the fact that they make iterative attacks (which normal natural attacks don't). INA is a perfectly legal choice for UASs in any circumstance, assuming you meet the +4 BAB minimum.