I should not admit this, but I used to do this frequently. I didn't flaunt it, nor did I tell the staff. Sometimes I'd just buy a 20oz ginger ale just before boarding and pour it in before boarding, sometimes I'd ask for a full can of soda onboard and pour it in that, sometimes both. Never an issue, but I was careful not to advertise what I was doing.
Edit: also, liquor stores are amused when I bring in a quart size bag and tetris in as many of the tiny bottles as I can.