This is quite cool, guest! The only thing, of course, is that levels created by this aren't exactly the easiest to share, but there's nothing really you can do about that (at least, I don't think so ).
I guess, what I should ask is... is there any way to change what .dat file the special graphics level point to? (Somwhere easy in hex...?) And, if so, does the file name have to be VGASPEC#.dat...?
Well, right now with CustLemm, you still have to copy the levelset you want to play to levelpak.dat, so it's just a few extra steps to copy the VGASPEC# files. So for distribution, what you can do is to name the VGASPEC# files you will be distributing to have similar names to the levelset, so that the user can put the files in the same directory amongst other people's custom VGASPEC files without overwriting one another.
I can certainly change CustLemm to look for any name (well, 7 letters or less) plus a single digit. But then you'll also have to keep multiple copies of CustLemm around.
As for changing which .dat the level points to, well the level stores the info that picks the "#" part of "vgaspec#.dat". What I can do later tonight is to release a set of blank levels that covers # from 0 to 9.