Hey bud. I've worked in this industry for 5 years and have never come across a Digital-RCA cable. It's weird that you guys have Digital Coax on your TV's, all the Sharps (Actually all HDTV's) in Australia run Digital Optical out, but they also have RCA out as well.
The only solution I can think of is probably a cheap HTS like you said with a dedicated receiver. I've heard bad reports on the decoders losing sound intermitedly so I'd probably stay away from them.
Good luck anyhow!!