I dont have an easy solution for you, but you have a solution you mentioned before:
Edit overscan.prefs.
To find out the right offset, just alter it one time to 704 and another one to 706, and only those parameters, save in both ocasions the resulting overscan.prefs file, and then compare both files. The changed values are the offset you are looking for.
Use filex from Aminet to edit the binary file.
