Have you got anywhere with this yet mcnobby?
I think i might have found some things that might come in handy for porting this over to the ethercard library...
Migrating from EtherShield to EtherCard library:
http://blog.thiseldo.co.uk/?p=623
Art-Net user guide:
Here is a project using OSC to control DMX via Art-Net that could be handy:
http://oschub.asia/weblog/?p=165
I'm a complete n00b with this stuff so i'm learning as i go (i hope). This started off as a simple DMX over USB project but has now grown into an attempt at an Art-Net node which is more than i bargained for!