This threat has gone from maybe "what" to something much worse. The original post was about being 500 kg to heavy for the assigned runway. Now how to you kill 500kg, not do a re-dispatch or cruise at a lower level and fly at a higher cruise, to be legal for departing on the assigned runway. I'd never recommend riding the brakes on taxi out, who wants to abort with possible overheated brakes. 500kg or just over a 1000 pounds is nothing. On a wide body 500kg is maybe an extra ten seconds at a higher power setting prior to brake release. Next time you roll down the runway note the fuel burn and you will have your answer.