You can try it, but I honestly don't think it would work.

He doesn't need to ask for your permission once he has established he has probable cause you've been committing a crime within your car that would justify him searching it.

He thinks your drunk, he now has the power to search your car to look for any beer with or without your permission. Finding the knife would be legit because he found it while searching for something else.

I've also heard alot of rumors about 3" or 6" or whatever. I'm not sure what to believe, but I'm going to try to look it up. A rule of thumb I've always heard if you're carrying a baseball bat in the trunk of your car, put a baseball glove back there with it, when asked you can say you were playing a game earlier in the week and that's why it's there.

I'm almost positive he doesn't need to ask your permission to search the car.