The pre-sales option for BT is £245. That said, i would imagine it could be done (but not sure if Seat would do it for you), but i think it may cost £245 or more !. If it follows the crazy situation with the USB option (which is £93 pre-sales), but if you want it fitted after sales then you pay £210 !!, then it wouldn't surprise me if it costs more.
Why don't you just get a bluetooth car kit from somewhere such as Halfords ?, you can get the parrot quite a lot cheaper than the Seat radio BT option.
Regards,
Andy