> 1) I wrote a tar to tape (very big)
> 2) The tape got rewound
> 3) I accidentally _wrote_ an empty tar file to the beginning of the tape
> Is there any way to read the old tar file?

Nine-track reel-to-reel tape?  Sure, "fsf" over tape markers until it works.
QIC tape of any type?  BZZT!  You lose.  Unfortunately, not only does writing
data at the beginning of the tape destroy large swatches of data (because the
erase head, active only on the first track, gets the entire width of the
tape!), but conventionally the drive microcode is set up to understand that
the EOF mark means the END OF FILE, end of story.