Thanks, hairy_hen!

For anyone interested in upmixing to 5.1 here's a small script you might find useful.
It uses avisynth (so needs that installed) and is based on soxfilter - Farina.

Extract and put one of the pcm's (DC48 DVD.wav for example) in the same folder.
Run the __Run_WAV_to_5.1_Multichannel_WAV.bat - and wait....
Split the wav with BeSplit like
besplit -core( -input "dc48 dvd51.wav" -prefix chn -type wav -demux )
or similar..
The channels should be in this order after splitting - FL FR FC LF BL BR.

Now ofcourse you would probably replace the lfe with hairy_hen's and encode to ac3 or dts.