It would be useful for trellis plots to be able to configure a more sophisticated look up mechanism than is currently available in creating reference lines from a secondary data table.
Currently the only solution that seems to work for this particular use case is through using column properties, so that each variable can have its own set of reference lines, but managing (creating and setting) thousands of column property values could be avoided completely by allowing a more sophisticated lookup mechanism from external table than that is currently possible.
Idea TS-I-9329 may be related as well.