Updated OP with what I came up with. I wasn't able to make use of $GIT_REFLOG_ACTION -- for some reason it was blank in every case, but reading the first line of the existing commit message, if it exists, does the trick.
I do foresee a potential problem if you're doing like an interactive rebase for example, and you go to edit a commit message that starts like the default "Revert " style--that could be surprising... Maybe some other cases I haven't thought of too, but yeah, works for me. Thanks for pointing me in the right direction!