Took in the 2006 GS300 for the second time now, as the creak has come back, worse than the first time. It showed up again with the arrival of the cold weather.
There are a lot of people who have creaking dashes, so it's a surprise the dealer hasn't dealt with the situation before. It's so common I think that the dealership is still waiting on the replacement cowl (about a week now for the part to come in -- part shortage anyone?).
I would find a location near the dealership with a stretch of road that produces the sound from the dash, and then ask to take a tech or someone for a ride for them to hear the sound.
Good luck. You can find the TSB as well here, something about windshield shims I believe to fix the creak.