BUDS port wire length

CAN BUS, properly terminated using shielded twisted pair cabling is easily good to 20 meters (max length at full speed is officially spec'd at 40 meters...) 

Agreed, but that is the length of the main bus, which is terminated with a 120 ohm resistor at each end.  There is no terminator on the end of wire going to the maintenance port, so in theory that would be considered a stub and limited to 0.3 meters. 

I realize the question is probably academic since the maintenance port is only used for diagnostics, but I was just wondering if an improper cable length could account for errors when using the maintenance connection, which then get logged as "Can Bus (diagnostic)" errors and show up in the fault log.  Or possibly even errors in the CAN when not using the maintenance port.  I can personally verify that when I get an error while downloading a log, that error will then show up on my engine fault log.

The Maintenance CAN Bus is Terminated at the ECU!
Measure from CAN_HIGH_2 to CAN_LOW_2 on either HIC Connector and you will see the 120 Ohm Termination as expected.

The Devil is in the details...

The proper way to implement the Maintenance Ports is to INDEPENDENTLY extend the HIC connections as needed to an interface location.
Then, when needed, a "Y" cable connection connects the two Ports together AT the Dongle.
The serial side of the Dongle then can be extended as needed to reach the computer.

What you now have Electrically, is a linear BUS extending from CAN-A to CAN-B and terminated on each end at the ECU with the Dongle located in the middle of the Bus on an extremely short (~2cm) stub connection.
If you keep the round trip length from the ECU_CAN_A back to the ECU_CAN_B to under 20 meters, you will be fine.

What you DO NOT want to do is to connect the CAN-A and CAN-B cables together and then extend the joined Bus any significant (30+cm) distance to the Dongle.
This will give you grief!

Bill, what you so perfectly described is the way I installed my ports, but I was thinking of this installation as two separate buses, and as such each seemed to be missing the second 120 ohm terminator.  Now that you explained this, it seems obvious that the Y cable completes the loop and yields a single 60 ohm circuit.  

It never occurred to me that the external Y cable would be a required element for the CAN bus to be complete.  So that must mean that this CAN bus is isolated for maintenance purposes only, since it is not valid until the Y cable is connected.  

Thank you.