WDTV seems to use the Sigma Designs SMP8635LF (took some googling to find that)
So you would have to contact them I suspect. I don't think Western Digital have the kind of access we are talking about. The good news is if you speak American then the should be able to communicate fine. The bad news is that I don't think you will get anywhere without a large potential order to discuss.
I happen to have the SMP86** linux SDK and its a long way from the kind of access you would need to add a new codec.