With a little bit more persistence I found you can change it with:
myDoc.sections.pageNumberStart = myNum;
It wasn't easy to find though, and the data browser in the ESTK wasn't much help :-(
I shall give that a try.
@John – it's worth more than just give it a try.
I rely on it, because the ESTK or its DOM documentation is, to say the least, very slow and sometimes you cannot get any result with a search, because: "there are too many objects found". Whatever that means ;-)