Awesome, and I see you're already upstreaming hooks. But I think you can
simplify this because every reply buffer has a hidden "In-Reply-To"
header (can be shown with `message-widen-and-recenter`). You should be
able to attach a hook to `message-setup-hook`, I think? Ideally you'd
use `message-header-hook`, but that doesn't appear to be run by notmuch.

So, something like should get you the ID of the message to which we're replying.

    (save-restriction
      (message-narrow-to-headers)
      (mail-header-parse-address-lax (mail-fetch-field "In-Reply-To")))
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