Using unused pins on an MCU to ease routing
I've done this before, and it does require some precautions that you set those pins to inputs on the MCU.
Other than that, most MCU pins can withstand short circuit for a short while.. so I think it is survivable if you set them up incorrectly during development.

Alternatively what you could do, is add soldermask to the pads that you use for snaking traces like this. That way the QFP pin doesn't get soldered to the board.

However, not sure how well this trick will hold up if you plan on mass producing the board.