You could sync over your local area network, and write an app to interface with the Zune's networking APIs. I've been working on a program that connects to my WLAN and streams music from my computer (currently, it can only stream one file, because I haven't
figured out how the XML parsing APIs work yet).
The ZuneHD stores music in the /Flash2/Content folder
The /Flash2/Content/Artist folder stores Album Art
The names are maintained in an internal database that is inaccessible by developers.