I would not do this in a modern car. Seurity systems will often respond in an undesired manner to variations in resistance between the key and the cylinder, and graphite coating can change the resistance.
I suspect also that you're confusing graphite lubricant with carbon. Carbon is used to create resistive connections, but graphite is, I think, completely nonconductive. A powder dusting may still allow some current flow, but the graphite itself is an insulator.