In this case the only solution is an external watchdog IC which resets the microcontroller.

The Internal watchdogs (actual watchdogs, not timers called into service) I know (Philips P89...) work just as well.

I solved a problem with a circuit needing software activated reset simply by inserting a P89C51RD2 instead of a Winbond chip and using the internal WD to reset.

Have fun,

Erik