If your source is LPCM 5.1 and you plug your HDMI into your TV your TV would need to be re-encoding that source to DD5.1 on the fly and then sending it to your receiver. I could see if the source was DD5.1 and it just passed that same signal along with no need to re-encode. Optical cables do not have the ability to transmit a LPCM5.1 signal. Due to bandwidth constraints they max out at PCM2.0 or use compressed standards such as DD5.1 or DTS. Often people have receivers that when fed a stereo signal do a very shitty up-convert to multi channel but retains none of the positional audio characteristics because it is just extrapolating a stereo signal.
Yeah, I should have been more clear. It can pass through 5.1 DD, not PCM.
No, it can't.
So I don't get why no optical is a problem. Wii U would need to support DD first, then you can worry about an optical connector.