Just let the mechanic know when you drop it off. He can certainly 'engine brake' during his test drive.

Before you take it in though, you may want to try it yourself. Find an area that is similar to where the mechanic will be driving (flat). Get up to speed (50mph) & shift from D to 3, then from 3 to 2. Come to a complete stop and shift to P. If you can't recreate the symptom then your mechanic probably won't either.

On the other hand, he may not need to. He may be able to diagnose the problem without recreating the situation.

Have you tried to run a tank of gas with a bottle of fuel injector cleaner added? Sounds hokie but worth a shot.