I have new information, but you're not going to be happy. So apparently it's not a "special dongle" like we were originally told. Supposedly you just need a regular USB-to-RS232 adapter to plug into the USB port on the TV. BUT...and here's the rub...it's not enabled in the current firmware release.  It's supposed to be coming "in a future release", but we didn't get any information on when that would be.

I've had to drop back to IR blasters for now, and have moved to ordering the MU8000 series that does have the Ex-Link port.  They're not that much more, and actually come in an 82" version anyway.

Fortunately we only ordered five of the MU6300's before we found the issue.  I had another requirement for a room that doesn't need RS-232 control, so I ordered an MU8000 and will be able to swap that in for one of the MU6300's.