I've spent several hours trying to get rapid override working. I happened to read about the change in the designation from MVO to RO. The Mesa Config program doesn't recognize this yet.
Then I noticed that the manual has the wrong name.
Instead of rapid.rapid-override, it should be rapid-override.
I have now entered the following manually and it finally seems to be working.
# ---max Geschwindigkeit Neufestsetzungs-Signale an Mesa Encoder - mpg (rapid override signals to mesa encoder - mpg)---