If you want to hack and change the e-mail address for say just one event, then you could drop into terminal and type cd ~/Library/Calendars and edit the calendar. Find the event (using the SUMMARY field), and then edit the line:
ATTENDEE: mailto:Change the e-mail address to what ever you want. Make sure that iCal is not running when you do this!
[Editor's note: This seems like a lot of work to send one reminder to another email address, but it works. It should also be possible to set up a rule in Mail to forward an incoming iCal alert to another email address. You could then use some rule logic to identify the type of event and send certain events to certain addresses and other events to other addresses. This is all conjecture on my part, of course!]

