The 680 is a GK104, which means it is initially based on the GF104/114 chip (and further developed from there, of course), so concluding from that, it should bitstream just fine. The only chips that didn't bitstream so far were the GF100/110 chips.
I have it in my gaming PC, so not connected to my AVR, but i can probably connect it for testing.
Regarding 4K decoding:
The next version of LAV Video will support 4K decoding in CUVID mode without having to hack stuff together, however the hardware performance is somewhat limited, it only manages at around 37-40 fps on the sample i tried (its a rather complex file, so it may be higher on others). I don't think its a deal breaker, 50p/60p material is rather rare at 1080p already (Blu-rays only go up to 30fps)
DXVA2 modes are not yet available, possibly a driver problem, they just crash.
3840x2160 is commonly known as 4K, even when its not an 100% accurate description.