I am not an electrical engineer or EE student; I'm a diagnostic technician. AFAIK, a CAN transceiver receives bits by measuring voltage between the network and its ground/low reference (high impedance, minimal current flow?)
When it transmits it pulls the bus to its dominant or recessive logical voltage levels. This could be 3.5 volts. The CAN high and low lines being connected by 60 ohms of terminating resistance could cause a current flow through the CAN transceiver.
The voltage drop over the line could then indicate a transmission is occurring.
But if the transceiver has a high impedance on the low side the current flow will be extremely minimal. The transceiver has to supply enough current to maintain the logical voltage threshold with as much as 50 other nodes sampling it and a 60 ohm short between the two.
I don't know, I just can't tell if it would work.
It's useful because CAN nodes sometimes fail to communicate or to send a message when an event occurs and without detailed introspection of the bus traffic this is opaque. Even if the traffic was decided there is no source address to most communication, making it further opaque where a failure to transmit occurred. CAN networks are extremely busy with general chatter even when a node is inactive so it can't be seen when one starts and stops transmitting.