Sounds like "Credit Hold" to me,  
 
When we get an order from a customer who is behind in payments, we enter a QUOTATION instead, and in the NOTES we write:
   
"This Quotation will be converted to an Order when your account is current.  Please advise your Purchasing Department of the status of their PO"    Then we FAX it to the customer's Accounts Payable Department.   99% of the time we get the desired result, they pay up and we make another sale!
 
This also provides a good way to keep track of their order in DBA...
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Actually, the next update will have exactly what you are looking for.  There will be a setting in defaults for "Check for open invoice in SO-A greater than XXX days".  if the number of days is populated, then a warning will be given at order entry if the customer has any open invoices older than that number of days.