Que yo sepa hacerlo directamente no se puede ya que es una característica del cliente que no se puede como tal sobreescribir
Microsoft dice
*"Regarding script, there is no "target='tab'" feature or any direct access to tabs from script beyond what is available with multiple windows today. We are working on balancing the default behavior for whether a window opened from script opens as in a new frame or a tab."
referencia
http://blogs.msdn.com/ie/archive/2005/05/26/422103.aspx
Salu2