Sorry this one has taken so long--it, along with some other feature requests, has been stuck on needing a change in the admin panel, and that's still an ongoing project.
Because the underlying change is so simple (two lines of code added), we've gone ahead and implemented it. However, we figure some people's scripts may not deal well with having a new variable unexpectedly show up, so we're only activating it on a per-client basis.
SO: If you'd like us to start reporting this variable (I've named it "ej_txn_id") for your account, please post here explicitly requesting it, or email email@example.com, and we'll turn it on for you. Keep in mind getting it turned off will require the same process, so you'll want to make sure your scripts will handle it properly first, or the change could break all of your notifications...