How do I tune E38 part-throttle timing on load-bearing dyno?

The Banish book suggests holding the dyno at a steady speed and adjusting spark to maximize instantaneous torque. However, with my G8 I don't have the ability to adjust the timing in realtime. VCM scanner looks to allow a subtraction of timing, but I could not observe any changes in it when I tried it. So my question is - is there anything I can do instead? Is it valid to shut down the run, flash in some new timing values, and repeat? I don't think so. Perhaps if I got the throttle position sensor back exactly where it was on the previous run and then compared the instantaneous torque to the previous run....? Thoughts?