I got th spaced dvd and you are right I, I havn't watched the comentary yet though.
I was chatting on dvdhelp.com and as of yet there is no way to have multi VIDEO streams in a file, as it woudl be cool to have a single file with all the xtras...
night.of.the.living.dead.-.is.basslined.avi
extras.night.of.the.living.dead.-.is.baseelined.avi
but it can not be done

....
Is there a video app like foobar for mp3s that can read from rars and zips? So you can have all the extras as a single file in a archive?
I think spud had it right though, filling the space with extras.... though many of the movies do not have them released.