While looking through a trade magazine, I found some hardware that looks useful. The "Pick-N-Hook" works with corrugated plastic - you insert it into the opening between the ribs, and you can tie on fishing line, string, or whatever your cable is. The "Twist-N-Hook" works on foam board or gator foam. Seems like it would be useful for temporary displays, portfolio presentations and so forth.
Please note that these hardware units are not rated, are not approved "rigging hardware" and should not be used in any situation where the item hung could pose a danger to someone underneath if it fell.