So far I don't think anybody has discovered a tool for extracting/creating the .pak files in the demo. The existing tools for Quake, etc all report that the RTW demo files are not valid pak archives, so I suspect that either it's a very obscure pak format or CA invented their own.

You can however edit the file using a standard hex editor, although some can be quite cumbersome to use for the editing required. I initially used my old hex editor, XVI32, but I found that for me Vedit better suited the part-hex, part-text nature of the pak files. You do have to pay for Vedit, but there is a 30-day trial download you can check out. Hopefully the full release of RTW won't use pak files, so there'll be no need for me to buy Vedit.