Add the fields  to the pages document library and then set them using a
calculated field.

I might be missing somthing here.

I guess I don't see in there how to add  a field  so that it shows up on the
create page  page.
Just to be clear, I am trying to add fields  to the page located at:
http://servername/_layouts/CreatePage.aspx?CancelSource=%2FPages%2Fdefault%2Easp\
x

The only way I see to work with those fields is to do it after the page is
created, not at create  time.