Monitoring voltage state without interfering
Cons: I would have to run the signal line throught a 1k ohm resistor and into the optoisolator. This would add resistance to the signal line AND prevent bidirectional signals.


This is not correct. Tee off the 5 V control line and place the current limiting resistor in the tee, not in the main line.







simulate this circuit &ndash; Schematic created using CircuitLab

The 5 V control signal will have to be able to source enough current to drive the LED, or you will have to buffer it.