Update, September 2017:
having finally reared adults I see that this could be either Mompha circumscriptella or M. brevivittella

February 2016:
(off and on) Since 2005, when cleaning seed of Evening Primrose, Oenothera biennis, I have sometimes found a small hole in the seed capsules and/or the seeds inside the capsules have been tightly woven together.

On the Microleps website T. Harrison says: "The larva of Mompha circumscriptella is a borer in the fruit of its host, with pupation occurring inside the fruit. The infested fruit does not show any externally-visible evidence to indicate the presence of the M. circumscriptella larva, and the best strategy for rearing is simply to collect and cage entire clusters of fruits."

Seed pods were collected in November 2015 and were separated this week (February 2016). This time I set aside some of the capsules to photograph. One of these years I'm going to collect seed pods WHILE the larva is still feeding and BEFORE the moth emerges.
One of these years.