Well, you have to have the files first to make the dat.. Once you have the dat, then you can run it against demos you already have (well, copies perhaps) to rename them to files that are in the torrent, move them to the directory of your torrent download, force a filecheck, and that will mark the ones you already have as downloaded, if some don't match, they get written over with 0s until you download them.
I have started the torrent, in the meantime (I've set uTorrent to append !ut, so if I get a dat, and start the torrent again, everything without !ut is a completed file.. I digress a little..) and I have noticed the directory structure might cause a problem, perhaps someone more knowledgeable could help.. But the directories are (and I do like the idea for Demobase, not knocking it) ...\Amiga Demobase\Amiga Demos\(groupname)\demos.zip
Not sure if that effectively kills a dat or not.. Would there have to be a dat for every group (ugh)? Hm..
Anyway, just trying to help speed things up.. I really want to check this out.