The following was copied from a post on our forums by Joe Rumsey(a.k.a. Mr Ogre), one of CTP's programmers
Actually, v1.1 does give you the ability to override files for scenarios. The way you do it is by making a copy of the portion of the ctp_data directory you want to modify inside the scenario save directory.
Let's say you have a scenario named "Whee". The save directory for Whee is ctp_program\ctp\save\games\Whee. If your saved scenario is named "mygame", it will be found at ctp_program\ctp\save\games\Whee\mygame.
Now, if you want people who load up mygame to have a modified units.txt, you'd make the directories:
and then place your own units.txt in ...\Whee\default\gamedata\units.txt
Now when they load mygame, that units.txt will be used instead of the standard one.
This works for EVERY FILE normally found under ctp_data. The game always looks in the scenario directory first, and if it finds the file it's looking for, loads it.
You'll see that this means you can distribute your scenario by simply zipping up "Whee" and having people unzip it in their own ...\save\games directory.
Now, what I was actually talking about with mod-packs is the problem you may have already spotted with this setup: It requires a saved game. So you can't do this trick with random maps. That will be in the next update. But for games with fixed maps, v1.1 should work fine. The ability to use random maps is what I'm using to define "mod-pack" vs. "scenario".