Looking for external watchdog chip for Raspberry Pi 3 model B board
The "about to fire" indication can be implemented by delaying the reset pulse. 

I suggest using a small microcontroller together with reset chip. The internal separately clocked hardware windowed WDT on the MCU can keep the micro honest and then you can use that to implement the reset. For example, you can implement a windowed WDT. 

The implementation should be easy enough for anyone that thoroughly understands both microcontrollers and watchdog timers. It won't be much more than a couple of chips. 

I assume you've already considered the RPi internal hardware WDT and found it wanting, but you could combine the two to increase the likelihood of a reset occurring under any conceivable circumstances.