I've made my first release of doom-data. you can get it from debian.halfcoded.net, either via apt (by following the directions on the page) or the constituent binary/source packages in binary/ and source/, respectively.
Anyone who is interested in helping out here could do me a massive favour and try the package out. I have modelled it fairly closely on quake2-data, but I'm not sure this is the way to go about things. debconf isn't designed to be used as part of code which does stuff. You are supposed to use it to ask questions that set values that you can read later on, in code that does stuff. So, here we're asking for the path to your DOOM2 CD-ROM at the debconf stage, and using this value in the post-installation phase to find the WAD file.
So if you supply an incorrect path (or it isn't mounted when the postinst stage begins) the package will fail to be installed properly. Issuing dpkg-reconfigure will not help you either: the state the package is in prevents this from working. You are forced to --purge it and re-install.
Removing the package also highlights another issue: the `filesize' is in the 60KB region, but the DOOM2 IWAD file is 15MB. It isn't counted in the package size tally.
In short, it may take some rather drastic re-structuring in the future. But in the meantime, please give it a try and offer some feedback!