Thank you! Yes, the zmmailbox command seems to allow all the sharing we need. So for example if Joe requests to share his Trash folder read/write with Mary, the command would be:
Hopefully the number of our people who need this will be small, so that we won't have to handle too many requests.
zmmailbox -z -m joe@domain mfg /Trash account mary@domain rw
Still, it would be nice if sharing the Trash & Drafts folders (or even the entire account) could be accomplished by the user himself in Outlook/Zimbra webmail, just as is possible with other folders. I will mark this thread solved, but perhaps submit an RFE for that.