While Boeing may allow operation like an overboost by not prohibiting it in the manual, the design of engine FADECs (controller) will not allow it under the regulations.
Even if the manufacturers can not prohibit this operation entirely, the pilots ability to operate outside this condition is like reprograming a computer chip in a highworkload environment.
Of course all the above is mute for discussions about non-FADEC engines like the B737-200's