Protecting your odometer during long stationary powertrain tests is definitely the right approach. I had to do a similar multi-hour test when checking a new transmission cooler setup on my track car. I picked up a Mileage Blocker from a company called Dynotest https://dynotestpro.com/ and it was the perfect solution. The hardware module plugs directly into the factory network and blocks the mileage accumulation while the wheels turn. It is completely stealthy and leaves no digital footprint in the car's ECU, which was a huge priority for me. It uses OEM-style connectors so there is no messy wire cutting required. It is a very solid, professional tool for serious automotive testing.