I had success on RS485 by replacing the line-termination resistor by a resistor with a smaller value in series with two anti-parallel LEDs, so that the current drawn remained the same. Might be worth a try, but I can't guarantee it would work satisfactorily at USB speeds.
USB data lines operate with carefully controlled impedance and timing, especially USB3 and are far more critical than RS485. Any loading other than a proper USB transceiver with have a detrimental effect on throughput and error rate.
A better question would explain why you need to monitor activity and what the specific circumstances are so that viable alternative approaches could be suggested.