Hi,
I've had the same thing happen a couple of times to me when my iriver has frozen (when I was using an old Rockbox build, doesn't happen any more). I know you say it won't open in any audio editing software, but have you tried Audacity then File>Import>Raw data? This worked for me both times without having to repair headers and I know that someone else has recently used it to 'save' a show.
Edit: I should add that I'm assuming that you can successfully copy the file to your PC. If you can't, then that's a different problem and could be to do with a hard drive problem on the iriver - I've had this problem as well and had to run chkdsk in repair mode to recover the file. I've now always split files into manageable sizes when recording using Rockbox's automatic split file size feature (I use 512 MB splits ) to avoid dealing with large file transfers - the files are seamless and can simply be joined using shntool.