> What about the behavior of (modern) caching resolvers, at start-up time,
> when they prime themselves based on the root hints file?

they are in the minority.

> What do they query for, i.e. "." with query type of "any"?

qtype SOA or qtype NS.  never qtype ANY, which is at best a diagnostic probe
and at worst makes you fall back with TCP after truncation just because of
some TXT RRset you didn't expect.

> If that's the case, then such resolvers will already have the answer (empty
> though it may be), and no new traffic should be seen.

that's not the case, but if it was, it would be true for only a minority.

> If I understand things correctly, that is, and some quick local tests I did
> seem to point to this behavior.
> 
> (Even trying to do "dig +trace" on the A of . seems to short-circuit
> locally, without querying any root servers.)

"dig +trace" is not indicative of any cache's behaviour, as you know.
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