[quote user="Andrew Womsley"] It looks as though I'll be iterating through every element in the attachment [/quote]
That's simple enough: create a ScanCriteria object and set the element type to find shapes or cells or however your border is composed.
Dim oCriteria = New ElementScanCriteria oCriteria.IncludeType msdElementTypeShape ' Or whatever ElementEnumerator oElements = oAttachment.Scan (oCriteria) Do While (oElements.MoveNext) ... Loop