You can add/query document Variables quite easily with VBA. For example:
Code:
Sub Demo()
With ActiveDocument.Variables
If .Item("MyVar") Is Nothing Then .Add Name:="MyVar", Value:="MyVal"
MsgBox .Item("MyVar").Value
End With
End Sub
You can then use a DOCVARIABLE field in the document to retrieve its value.
Word's custom document properties are about as easy to work with, but built-in document properties require less work, since you always know they'll be there. Both kinds can also be viewed via File>Info>Properties and can be referenced in the document via a DOCPROPERTY field.