Create a separate design just for the first page of chapters.
Let’s say you want the first page of each chapter to have a 2in top margin and no header. First pages use the body-first page master. The body-first page master uses the region-body.odd attribute set to define the body region, which includes the top margin (along with the inside, outside, and bottom margins as well, of course).
You don’t want to change the margins on body pages so you can’t edit region-body.odd. What you need to do is define a new attribute set and use it to describe the body region on the body-first page master.
<fo:simple-page-master master-name="body-first" ...