Hmm. Maybe first determine if the file is writeable, as you've already done, then if it isn't, make a copy of the file and name it incrementally. Have your game only use the most recent file and also attempt some kind of housekeeping to delete any old files it has permission for.
A bit messy and hacky I know, but it's the only thing that springs to mind at the mo. :P
A bit messy and hacky I know, but it's the only thing that springs to mind at the mo. :P