Is it safe to do the hard reset, (touch disconnected car battery cables together), on the MHEV system?

TIA

It's not safe on any modern car.
You're literally shorting out the electronics and you have potentially high currents flowing in ways the engineers never intended. People get away with it, but then you'll probably get away with stopping your car by banging it into park every time too. Doesn't make it a good idea.

There are capacitors holding charge and keeping electronics alive when the power is cut. It's these that you're trying to discharge but doing it by releasing all the power in one big lump, across circuitry that's not designed to handle it.

You can achieve the same effect safely by disconnecting one side of the battery and simply leaving it for 30 minutes. Let the capacitors and other components discharge naturally. 2021 L494 D300MHEV Autobiography
2019 Discovery Sport D180 HSE