> Does this feature have a command for it on the serial port??

Probably not, but it's a reasonable suggestion.

> Does this mean that these features are not available when I only have the
> Kenwood remote??

Yes. The Kenwood remote doesn't have press-and-hold capability on all of the buttons, so some features are available only from the Rio remote.

> Does this work for the MKI as well??

Hmm, I'm pretty sure that it won't. The Mk.2 has an extra chip that allows us to sample the aux and tuner sources.

Rob