Would it take much extra wok to incorporate UIR/FIR boundaries in the FD display, possibly accompanied by a warning chime as this is crossed? All it takes then is one call to ATC, "Do you want us to remain on this frequency?"
Personal opinion; once an aircraft is intercepted I feel it should then continue to the designated airport. Thee is gong to come time when it is a genuine incident and the person advising Ops Normal is in fact the hijacker.