Not sure if this is exactly what you are looking for but why not just place the second line that needs to be on the first page only after the <?start:body?>
Header 1 Header 2 Header 3 init_field This is the row to be shown on all the pages
if_field This row should only be shown on the first page. end_if change_variable
Thanks. Coincidentally, I solved the problem in the exact same way as you specified.
The only problem, with this solution is that we won't be able to add anything to the header after that row.
But thanks again.