Thanks, Alex, I really appreciated the thorough explanation of the history here, which makes sense of things. To answer your question about why I was monkeying with *data-readers*, I don't think I understood at the time that data_readers.clj did everything I needed to do. For example it wasn't clear to me at the time that this file can be distributed with libraries and will work fine (I thought it was just something individual users made). I also didn't realize it works fine in the REPL, possibly because it took me a bit to get the syntax correct and in the meantime it wasn't loading in the REPL. (I was mis-naming my tags like namespaces, "foo.bar.baz", instead of like symbols, "foo.bar/baz", although I can't recall if this was an issue with data_readers.clj or just *data-readers*. It worked fine with edn/read to name tags that way so it took me a bit to realize what I was doing incorrectly.)
Anyway, it turns out I do not need to monkey with *data-readers*. Thank you again for the thoughtful reply.