I think this did the trick. Testing now...
Yup, this works perfectly. In the GUI it still shows as @zcs.mydomain.com, however when I send emails out they are from @mydomain.com. Once we migrate our current email system over to Zimbra then I can run a quick command to change the domain to @mydomain.com. If I do that now then I won't be routing emails correctly to users on our current (non ZCS) email server.
zmprov md zcs.mydomain.com zimbraMailCatchAllAddress @zcs.mydomain.com zimbraMailCatchAllCanonicalAddress @mydomain.com