Laura,
 
By any chance, do you use any of the SO's shipping reports to view ahead of time what is supposed to ship?  If so, just enter the words "VERBAL" in the SO Customer Order field and then add the field "BKAR.INV.CUSORD" to the shipping report RTM's you use.  As you are reviewing, any time you see the order "VERBAL" display on the report, you know you must get  hard copy of the PO.  
 
Another concept would be to add an "Alert" note (will only work if you have EVO Notes turned on) to the SO so that any time this SO is accessed for any reason (make changes, add hard copy PO #, Release for shipment, Etc.), this will trigger a note to pop up indicating something is needed.  If people are in the habit of ignoring note alerts, there is now a default switch that will not close the alert until it is opened to read.
 
Just some thoughts.