(Disclaimer: I work at Google and I worked on Google Mobile Sync , in particular the calendar, but in this answer and everything else I write on this site, I write as a person and do not represent Google.)
I do not believe that there is an API for public notifications. As far as I know, you will need to interview. However, you can, of course, request changes from a specific timestamp; it's not like you download the whole calendar every time.
On a secondary note: good luck! Of course, I did not appreciate how difficult the calendar was until I started working on it ...