I've played with the Demo before. The most important thing you must do is NOT make changes to the RFA files. Unlike the retail game, the demo retains the ability to run unpacked RFA files direct off the main game folder. Just extract them in the base game directory (be sure to preserve the folder structure).
Just keep in mind you can't add new maps as the map the demo came with is hardcoded in the EXE. So if you wish to use a different map, you must rename that map to Tobruk (for the singleplayer demo) or Wake if you are using the multiplayer demo.
The demo really hates modified RFA files so unpacked RFA files is the only way to go on this. And it does not supoort "patching" as a result so you can't add "_001" at the end of a rfa file as the demo won't work with modified RFA files.
So be sure to backup the original stuff as replacing it is the only way to go.
The demo can load things in a similer way that the debug exe of the retail game can. (unless the multiplayer demo works differently?) so if you know how to load unpacked stuff with the retail debug exe, then doing the same for the demo shouldn't be hard.
I have worked with the singleplayer demo and successfully made changes (I almost got El Alamein to work, but alas, only a really old version of the map might work as there is some AI code that is not compatible). I haven't tried to mod the Multiplayer demo, but I expect the method of modding to be similer. So I may be wrong, but am sure that I am right.
