Can you not get at it by popping off your side plates to your dash and then popping loose the silver trim strip that goes across the dash horizontally? If you open your door and look on the side of the dash, unscrew and pop off the piece. Then you will have access to the screws that hold on the silver dash strip. The silver strip comes off pretty easy with a little gusto but sounds like you just need to undo the screws on the far side of the dash for the silver strip and get to the vent behind it. I've had mine off several times doing stereo work and such. It's not full access to the venting but, it does remove the air director and get you to the actual piping behind it, sort of
Just a thought, hope you get it!