As I understand ArtNet, it‘s basically a DMX over Ethernet. So you get a bunch of value between 0 and 255, that can mean anithing. For Caspar to give them any meaning you would need a controller (Caspar client) that interpretes these raw numbers to commands. So based on the address you could define it to choose a videoclip and play it etc. So I would suggest writing a custom client, that receives ArtNet packages and convert them to AMCP commands via a kind of lookup, that can be defined in its user interface.
EDIT: That seems to exist already and is mentioned in the old forum. Will try to find it in my backup later today.