I don't have a solution off the top of my head but there may be altogether better ways of accomplishing what you're trying to do.
I assume that subcontractor.com isn't running Zimbra...that would make your life easier. However, is is Joe using Outlook? iCal? How about you?
Basically if you could get Joe to use a mail + calendar combo that supports publishing calendars, then it'd be easy for you to send requests to
joe@subcontractor.com, have Joe incorporate your invitations into his calendar, and publish his calendar somewhere that you can look at it.
(You might also get things working if Joe is using Meeting Maker or Google Calendar.)