I know you want to prevent kickdown from happening at a dyno-run, but disconnecting the linkage totally is not the way to do that. You should just do some short tip-in testruns to re-set the linkage so it doesn't downshift.
I've had my car on the dyno and with just 2 or 3 dry-runs the downshifting was taken care off.

Lowering the transmission's oilpressure and make it unsusceptible to throttle changes, just when you're about to do the most straining test of all on the driveline (just when you need the most clamping force/oil pressure on the clutches) isn't the healthiest situation for the trans.