It's early days and all that, but I thought you might be interested to know that I just decoded and displayed my first MKV file with AVC video in DGAVCIndexNV. I adapted low-level MKV parsing code provided to me by Haali.
There's still a lot of work to do, especially with DGAVCDecodeNV and audio demuxing, but it looks like there's nothing to stop me from implementing full MKV support. With DivX backing MKV+AVC now, I think it will be a useful addition to the DG tools.
In case you're wondering, I made my own MKV file with mkvtoolnix.