With the method I suggested, (using a subscription), you will have to do all the calendar updates with the google calendar interface. It will probably not be the best method for your purposes since I think you want to update the google calendar from the iCal interface.
I use that method for group calendars. I.E. I maintain a calendar for a non-profit organization. I send the url to the other members of the organization so they can import the calendar contents into their own calendar. By using the subscription method, ical will periodically check the calendar for updates.