[RoboChair]

Using magnesium would only pose an issue if you are using open flame in the pits(which you shouldn't as per the rules), or are producing arcs(which you shouldn't). Magnesium is not some super sensitive substance that will spontaneously light on fire, you deliberately have to try and ignite it unless it is a finely divided powder. Aluminum powder is flammable too. As long as you work with it properly and keep chips out of your electronics I see absolutely no problems.

I would bet money that you can't ignite a size-able block(greater than 0.25" minimum dimension) of magnesium with a blowtorch(not a welding torch) in under 15 seconds. Magnesium shavings or ribbon are a different matter but even ribbon still takes effort. Buy a Class-D extinguisher if you are that paranoid.