Can it be built in Linux? I ask because using WxWidgets might provide such portability.
Theoretically, with small modifications, yes. I use WinAPI:
1) To check that the file or directory exists. (It can be replaced by POSIX functions)
2) For reading the registry to get the path to the game. (This can be turned off when compiling not under Windows)
3) Dialog boxes(open file, save file, choose directory). (WxWidgets dialog boxes can be used)
4) To get a list of subdirectories and list of files by mask. (I think Linux versions of these functions are easy to write)
But is there a need for a native version for Linux? WINE is required to play Fallout, it can be used to launch the editor.