My recollection is that you can declare, as a FILE level attribute in the MFD, a fex to be run as setup before retrieving data. But I cannot locate syntax for the feature in the online documentation.

Is there such a feature? What is the syntax, and what restrictions apply?

I would like to use it to build a reference (FOCUS) file on the fly, and include it in the cluster join of a business view, pending availability of a suitable reference table in the database.

Perhaps you're looking for "MFD_PROFILE"?

FILENAME=table1, SUFFIX=SQLMYSQL, MFD_PROFILE=test/preprocessfex1, $

Indeed.

I couldn't find it the doc here (e.g., under FILE level attributes of mfd), because it's a 7.7.04 feature and we're still on .03

Interestingly, I first started using it on a v7.7.03 environment, with no hot fixes.