Hi Seth - process/object XML is held in the BPAProcess table in a single cell. If you copy an Action stage that references Set Cell Value into Notepad you'll see how the XML is structured, and that will give you an idea of how a WHERE clause could work in a query. Directly querying the DB is not advised though, unless you really know what you're doing and accept the risk involved.
A longer but less risky approach would be to use the Find References feature in the BP Client to find where the MS Excel object is used, then open each diagram and use the Search to look for Set Cell Value.