Indeed, tried that. Serial commands are not being received by the player in DC/Car forced mode, but are being received in AC/Home mode. Both cases with the rate at 115200 bps.

There's some discussion of it here:
http://empegbbs.com/ubbthreads.php/ubb/showflat/Number/144723
http://empegbbs.com/ubbthreads.php/ubb/showflat/Number/173872
http://empegbbs.com/ubbthreads.php/ubb/showflat/Number/183322

I *think* what these are telling me is that I need to enter Config.ini settings like this:
[Hijack]
suppress_notify=0

[Output]
notify=1