...synchronization with the RIOcar, since it appears that an update to support HD units...

Just because they're both HD products doesn't mean that it's the same protocol. Hopefully we'll be able to do it, but the car player is discontinued, so it's not going to be a high priority.

Support for currently-available, supported products (like the Riot) is higher priority.

So, one day, synchronisation with the car player might happen. Don't hold your breath.