Thanks for the response, but I wasn't actually talking about the new units themselves, but about the names that the game shows for new units.
For example, in the cfg file for the HQ it calls the HQ "army.building.hq1". But when you are playing the game, and you click on your bulldozer, the game will correctly show the name "HQ" in the build menu. I'm pretty sure that the english.dat file contains the information that tells the game to do this. If you download any of the ants clan maps that are linked in the video, you will notice that they install a modified english.dat file into the game, and this changes the names of multiplayer maps. (e.g., the file for the 8 Hanging Gardens map is named something like "8hanging.x", but the list of multiplayer maps in the game shows it's name as "8 Hanging Gardens"). I guess I need to correct myself on one point, though. These maps didn't actually change the names of units, but just of the maps. However, if once you knew how to edit the english.dat files, I'm pretty sure you could change the names of units in the game too.
I guess this may seem like a pretty trivial thing, but it would be kind of interesting to know how to edit this file. I know it won't enable anyone to make anything new as far as gameplay goes, but is does look a little nicer if the game calls your new unit "Super HQ 2" instead of "game.types.army.building.sphq2".
Thanks in advance for any help!