What about using a USB-to-Ethernet adapter? Is there anyway to get that working with the Rio Central?
Yes, in fact, I'm using one on my Central. You plug a USB cable from the central's rear-facing USB port into the USB-to-ethernet adapter, then plug the ethernet cable into the adapter. There's a trick: It has to be the right kind. If you go to the Central's "About" menu (I think it's the about menu if I recall correctly), it lists the specific models of USB-to-ethernet adapter that were supported. Might be hard to find one of them, though, that was years ago.
I can't for the life of me (except for cost savings) think of why they didn't include an RJ-45 jack and Ethernet support
Yeah, they were trying to save on cost. Fortunately they were able to code in the adapter support, so anyone who really wanted ethernet could use it.