Very Computer

    is this a usable idea?  will this work?  Or will drivers have to be
  written anyhow?

Tape drivers are character devices, because
1.  You want writes to go to the tape, in sequential order
2.  You need variable block sizes to accomodate what the
        tape drive needs physically.

Now, with a character device, you need to transfer data to/from user
space.  This may be non-contigous in physical memory, which means
you need either an intermediate buffer (You want to increase kernel
memory enough to accomodate the largest tape block?  I've used
100k block sizes before), or for the device driver to support scatter /
gather.

Problem : the current SCSI drivers do not support scatter / gather.  This
is being rectified in the rewrite.  After the rewrite is done, it should
be possible to write a decent SCSI tape driver.