Steady state tuning

Have you tried steady state tuning without a dyno by jacking the driving wheels off the ground and using left foot braking or varying handbrake pressure. Maybe also adding a large fan in front of the radiator to give some airflow over the radiator.

You'd struggle to provide enough resistance using the brakes for it to be useful and then you also need to manage the heat that you'll put into the brakes. There is also of course no feedback in terms of power and torque if you do what you're proposing.