Each Modern Quild Guild has it's own "definition" of what constitutes Modern Quilting...and by doing so, defies a broad definition. Do you have a lot of modern quilters in your area? Is there an MQ guild? Ask them for guidance. Send a PM to Holice and ask his advice, both from the judging standpoint and the MQ movement standpoint. He's very interested in it and is soaking up knowledge.
Many 'modern quilters' use very traditional patterns with modern fabrics and bolder colors. Where would you 'categorize' them? Are you sure you're ready to judge modern quilts separately from traditional quilts? They are not art quilts by a long shot. It is merely a contemporary style using mostly traditional construction techniques. Is there a separate category for civil war quilts or 1930's quilts? Just playing devil's advocate...not criticizing. Best of luck to you.