Maybe this year I'll be able to get a picture of the fly itself. I would think a fine muslin bag tied around the leaf would work.

The only reference I have is "Leaf Mining Insects" by Needham, Frost & Tothill from 1928. It is quite possible that the insects have evolved into new species since then ;-)

Yes, that is a fantastic reference overall, but the species names for agromyzids can't be trusted. Not to mention that hundreds of additional species have been discovered since then (about half of whose hosts are known). According to the specialist I contacted, "Much early work on agromyzids is flawed because they weren't looking at genitalia back then. Subsequently, species concepts have changed and so have many names. Misidentifications in the past have also led to considerable confusion in the literature." I had specifically asked about this species because "melampyga" used to be the species that mines in mock-orange, and I was wondering what the heck was going on.

If you can find a copy, the best reference for identifying agromyzid fly mines is:

Spencer, Kenneth A. and George C. Steyskal. Manual of the Agromyzidae (Diptera) of the United States. United States Department of Agriculture, Agricultural Research Service, Agriculture Handbook Number 638, 1986.

My impression is that flies are easier to rear from leaf mines than other insects are, because they develop fairly quickly. I collected some winterberry leaves last summer to try to try to rear a leaf-rolling moth larva, and within a few days some mines appeared and then these flies emerged from them: