Sending emails to attendees in this case is handled entirely by iCal & Mail.app. Zimbra just delivers the mail. I've seen some iCal hacks that change some of its notification behaviors. A quick Google search turns up
iCal Reply Checker | John Maisey. It may or may not do exactly what you want, but it does point in the general direction of the type of thing you're looking for.
I have a hunch that some of the options set by this and similar programs are actually editable within the text-based .plist file used by Mac OS. You could try hunting around for those...or you might do some google searches for people who've already done so.