This issue is related to SEAMFORGE-253. Using the CommandInterceptor does not give me the wanted result as it relies on the ConsoleReader mechanism to read entire lines. I have had better results with using the ConsoleReader ability to register so-called TriggeredActions.
These TriggeredActions are simple ActionListeners that get performed when a particular character is read. The Forge API needs to include a new interface in the SPI package and the ShellImpl needs to register the beans implementing this interface during the setup of the ConsoleReader.