I have an Athearn Genesis F7A-B with the MRC sound installed.
I am able to both write CVs and read-back CVs using my Lenz system by placing a 10-ohm resistor in series with one of the programming track leads. I use the CV option in the Programming mode. A 20-ohm resistor also works.
Just thought I would share that with you so you can pass it on to your customers.