Sorry for the reply to self, but ...

I found something in Excel Help under:
- DocumentProperties Collection Object
- BuiltinDocumentProperties Property
- CustomDocumentProperties Property

Specifically:
"Using the DocumentProperties Collection
Use the Add method to create a new custom property and add it to the
DocumentProperties collection. You cannot use the Add method to create a
built-in document property.

Use BuiltinDocumentProperties(index), where index is the index number of the
built-in document property, to return a single DocumentProperty object that
represents a specific built-in document property. Use
CustomDocumentProperties(index), where index is the number of the custom
document property, to return a DocumentProperty object that represents a
specific custom document property."

SO: it appears that custom properties can be added and read using VBA code.

I have yet to figure out though whether a 'ContentsDocumentProperties'
object exists which contains sheet and range names (it appears not) and
whether any of this will speed things up.

Regards,
JvL