OK, I had some help, but I've figured it out. Let's assume I'm trying to calculate the date that a UDF was updated (say, U_USERFIELD). If I run a query on ACRD and look at the most recent entry I would find a NULL entered against the column U_USERFIELD. But then I look at the same field in OCRD and I find that there is a value assigned into it. This means that it is the most recent change that has been made to the BP: and since SAP 8.8 onwards, it will not be entered in the change log. So, the value I want for UpdateDate is the one in OCRD.
The full query will also verify that U_USERFIELD is NULL in the change log but non-null in OCRD. So it should be fairly easy to do an inner join on ACRD and OCRD and pull out the values that I need.