I thought he was thought up by the ad company that Coke hired. Although I might be wrong, I'm not a Headroom historian
Let's hit the wiki here:
"Max Headroom is the name of a fictional artificial intelligence, known for his surreal wit and a stuttering, distorted, electronically sampled delivery. The character was created by Peter Wagg, Annabel Jankel and Rocky Morton and performed by Matt Frewer.
The Max Headroom character originated in 1985 as an announcer for a music video programme on the British television channel, Channel 4, called The Max Talking Headroom Show."
Well damn, whaddya know. You're right, he pre-dated the coke ads. I don't know why I remembered it the other way around.