Dear Scott
in many place in SAP EHS SAP is saying: if you have assigned (by whatver reason) same material to more than one spec please use (function module X, BADI y ....) to get a "unique" spec.
Your topic is different. One approach is by using the extension capabilities in EHS. During save process you can call a "BADI" (if i remember correct please check customizing). Here you can check whether the same material is linke to two or more spec and "hinder" that the save is possible.
The same could be done on high level if you would use the "Status" in spec header; but the "BADI" is the right place.Check customizing and prepare th extension.
C.B.