I too have done it many times with no issues but for some reason Inspectors love to pick on that rule if it is done in a way that isn't a problem... but I guess they have probably seen it done in so many wrong ways that they don't want anyone making a habit of it?
I've had to add sub-metering equipment to existing services a few times, and the only way to "tap" the breaker or lugs was to place the wee little conductor behind the much larger one; think 14awg to power the metering equipment on a 200A service. I like to crimp a long-nose ferrule onto the small conductor and then place it behind the larger one before tightening the lug . I figure that way, the 14awg wire isn't really different than the individual strands of the large conductor. I also typically zip tie the conductors together right below the lug, so that some muppet pulling on the little conductor has less chance of inflicting damage at the termination.
