Firstly the specific servers and the clients accessing them can be chosen for their geographic position using a system such as GeoLite. Shapes, diagrams and letters can be drawn in this way to communicate a specific message above and beyond the mere fact of the attack.
Secondly the properties of the network traffic sent to the servers can be varied to encode a message. The timing of messages can be used to transmit values and thereby numbers, text, images and even sound or video although the latter would be very slow.
The problems with these schemes are that the variation of traffic volume involved in structuring the messages, the topography of the Internet and the effects of the DDOS attack itself would work to destroy the coherency of the encoded messages. This functions as a commentary on or allegory for the effectiveness of reasoned argument versus simple rage.
Attacks of these kinds could be simulated using virtual machines on a closed network. This would function as a proof of concept and as art. Capturing and visualizing the network traffic of the attack would serve to recover the intended message or to track its degradation, and again would function as art.