Do any of the Datalog DBs support the idea of a "custom index" - a user-defined method of sorting the values for the v part of the various eavt/vaet indices? I could accomplish something equivalent by reifying the index as values, but I'd have to do the work of periodically updating the index myself on changes to the indexed value.

Datomic allows you to do this with https://docs.datomic.com/cloud/schema/schema-reference.html#tuples. There are composite, homogeneous and heterogeneous tuples which covers a pretty broad set of use cases.