You're never going to see a version that will transcode AAC to DTS or AC3.
If your receiver won't accept AAC input, then all you can do with the AAC is output it as stereo PCM.
And I don't think anything other than stereo AAC is supported by any of the WDTV devices anyways, so even if your receiver does accept AAC, I have a feeling the WDTV won't be passing the 5.1 through to it.
Seems rather pointless to rip the AC3/DTS on the disc, encode it to AAC in a file, and then want to change it back to AC3/DTS for playback. Why not just keep the AC3/DTS to begin with, then there will be far less playback problems.