Artnet is a bit more than just simple UDP commands, it is a protocol with several versions. Generally it reflects DMX standards, so each universe has 512 channels. As it is used in across a variety of environments, hardware and software, these standards are kept quite formal. Other devices will not work if you modify the standard.
Another reason for the limit is that Artnet can (and was limited to in version) use broadcast, so this gives susbtantially more network traffic that just the message you are sending to one of many artnet receivers. All versions have had some kind of handshake implementation as well.
I am not sure where you found artnet implementations with more than 512 channles per universe but I would guess these will not work well with artnet devices.
The way this is worked around in the real world is to use multiple universes. Sometimes this is complicated and annoying for wiring, but pretty much all pixel mapping gear is ready to deal with this. Artnet version 1has up to 40 universes, however most of the software implementations I have seen limit this to 4 universes. Artnet 2 has 256 universes, and Artnet 3 and up can address 32,768 universes.
I found 2 ofxArtnet addons, I am sure there are more and each of these have many forks. Both seem to let you set universes for sending messages.
