No, I think SDK wouldn't work here since the DLL can only be loaded from within the engine but the resources have to be present during startup. I never thought of the possibility to have mods in an A5 game but the only way I can currently think of would be to have a packed file with all needed files (graphics, wdls...) unpacked at startup with your own starting tool.
Concerning the general un-bind problem:
I think it depends on how you give the name of the files in your code. There are two possibilities to give a name ("name.wav" and the other one with those "tag-brackets"), one binds the file the other one doesn't ("" doesn't bind a file I think).