Method 1:
Sounds like you already saw it but to restore to another account you add:
-ca -pre SomethingYouWantAsAPrefix
then
a) forward (less clean obviously-but you'll have the content back)
b) drag drop in thunderbird (connect to both accounts-with /tb on end to keep the original times)
c) or another thick-client etc
Method 2:
If you don't restore to another account:
zmlmtpinject -r
user@domain.com -s
sender@domain.com /opt/zimbra/location/#/#/msg/123-4.msg
Injecting a whole folder of files would be:
zmlmtpinject -r
user@example.com -s
sender@domain.com -d /opt/zimbra/location/#/#/
If you do this a bunch over and over in a short time you might have you'll run into the single instance storage/drop duplicatea fun. Though you can turn that off across the entire board by globally setting zimbraMessageIdDedupeCacheSize to 0, but I don't think you want to right now from the sound of it
Method 3:
zmmailbox addmessage
There might be a RFE in for a GUI backup browser/ability to drill down to an individual email...