Ævar Arnfjörð Bjarmason [off-list ref] writes:

We could surely add magic record types, but how would such a dance be
performed while keeping compatibility with existing JGit clients?

Yes.  It is exactly the point of the question I asked.  If it is
simple and easy to add such a new type that is ignored/skipped by
existing clients, then we can go that route.  If it is simple and
easy to add a new bit per ref that existing clients would not barf,
we can use that as an alternative implementation strategy.

And if neither is possible, and there is no other viable third way,
then what I wrote in the part you omitted from your quote still
stands, which was:

quoted

It is probably a lot more sensible to fail refs_create_reflog() and
safe_create_reflog() (which is a thin wrapper around the former), if
we cannot implement "a reflog can exist and have no entries yet"
semantics.