Ugh. After some testing and more reading, I see that the answer to my last question is:
Note Unlike all other validation options, the Not empty or Validated by calculation options are evaluated when FileMaker Pro attempts to each record. Therefore, these types of validation failures cause the entire record to be skipped. (All the other field validation options are evaluated when FileMaker Pro attempts to commit each field.)
So, I can't figure out how to do the following:
Import matching records into a database such that any blank fields in the source will not overwrite any fields in the target but that any populated fields in the source will replace corresponding fields in the target whether they are populated or not?