- open the deceased's eyes
- position the eyes so that Face ID thinks the eyes are "looking into" the iPhone
- pump warm blood/liquid into the deceased's face in order to fool the infrared temperate monitoring
Seems possible, but sounds like an awful lot of work unless you did something that law enforcement really wants to look into.
And from my spy movie expertise:
Turn the heater in the room full blast to achieve the needed thermal level (The Saint, Sneakers)
It's not that OP worries they might be doing something sketchy enough that law enforcement would go through those lengths. The issue is that Face ID's difficulty to crack is a function of time, not knowledge. Thus, it is not the holy grail we seek, and if you have anything of interest on your phone, whether political, corporate or just illegal, it's not even an option if you wish to secure your system.