File ops on a VFS mounted file

Hi,
I've got some executables built with kbskit on Windows and Linux, and
I'm trying to get one of them to copy itself to another file, as part of
initialising a new VFS for an end user application.
The VFS was created with vlerq by kbs.tcl.
When the exetuable VFS attempts to copy itself to the target image
(opts(1) == the target file name) -

  if {[catch "file copy $extraarg $argv0 $opts(1)" output ropts]} {
    ErrorExit 1 "init: failed to create target VFS \"$argv0\": $output"
  }

I get the error:

  failed to create target VFS "...": can't overwrite file "..." with
 directory

- because of course the VFS is also mounted and my script is running
inside of that.

I tried using [open ...] and [puts ... [read ..]] but the VFS image
file can't be opened while it's mounted.

I was able to successfully use [exec cp......], but external tools
probably won't be available on the end user systems and certainly not
on MS Windows.

But if the VFS is mounted then it's already been opened by the VFS
library itself. Perhaps there is a way of duplicating the file handle or
accessing the naked VFS image file without invoking the VFS library?

Kind regards,
Scott