I wondered the same thing. But I've seen estimates of operations at non-towered airports, so I assume the same rationale can be used for towered airports after the tower is closed. Here's a link to an FAA document on that subject:
www.faa.gov/data_research/aviation_data.../GAModel3F.doc
No, this link does not explicitly answer your question, except to say, the FAA probably does, and they're estimates.
I've seen estimates for operations at my own uncontrolled airport, and they've always seemed high to me. But when I compare them to other airports I know about the relative comparisons (higher/lower/way higher/way lower) seem reasonable, so I think these estimates may be of some value.