If the bypass plug is not 'set' hooking a filter to the IN-OUT lines would have no effect because there is no signal for the pressure to pass thru the filter. I am not completely sure, but, I would venture that the factory bypass filter drew off of one of the large ports (thru a restriction, like .030 or so...), thru the filter, and dumped it back into the crankcase like the earlier models. You are correct that bypassing the pressure thru a filter and dumping it directly back into the pan would kill the oil pressure and flow. With the early technology, and sometimes the absence of apparent logic, it's a wonder we have these beauties to play with.
Tim
