[tommy]

Artturi,
no, I don't think this is possible currently; as mentioned, this hack works on the MTA (Mail Transport Agent), which in zimbra's case is postfix.
The MUA (Mail User Agent), zimbra in this case, should be the sole responsible for implementing it natively and correctly, since it is the one that handles the graphic interface connecting to the user (and should ask the user whether to send a notification -outgoing- or popup the delivery notification -incoming).
In this case, postfix inserts (in a dirty way) the header responsible for popping up the notification on the receiving MUA (outlook, thunderbird, kmail, others), but postfix doesn't know how to handle it; it just inserts the header and sends it.

At the same time, if zimbra receives a "Disposition-Notification-To" header, at the current state, it gets ignored.

So, as I see it, you can only enable it - without final user intervention - for the whole domain or for certain persons in the domain, but it will be enabled for each and every mail going out WITHOUT ANY interaction with the sender, at least until an admin removes the instruction manually from localconfig.xml and delete the add_goofy_header file.
And this may work or not, or may present other problems which I'm not aware of yet. I only know it has been strongly discouraged (and I'll go on saying that) from postfix-users, due to being an MUA option, not an MTA option, since the MTA should not insert headers that should be user-decided.