decapsulating a MOSFET to create a bulk terminal
This probably isn't going to work on many devices, as in most devices you already have access to all the terminals. Certainly on power MOSFETs, which are vertical devices, there is no bulk terminal. The substrate/bulk contact, if it exists is commonly tied to source, as otherwise you would end up with weird behavior when using the transistor as intended. You'll find most devices already make electrical contact to the back of the die for the drain.

Note that I'm not even addressing the decapsulation aspect because you couldn't make the connection you want starting from a bare die part.

I have seen some devices where the bulk terminal is not implicitly/explicitly shorted and exposed to a pin. Advanced Linear Devices makes parts like this, such as the ALD1107. However, it is very uncommon to find parts where the bulk terminal is not internally connected to the source.