I'm currently searching what is the rml file generating the header in openerp 7. I can't find it... 
I have found server/openerp/addons/base/report/corporate_defaults.xml but no... Or maybe there is a cache caching the rml befort the report generation ?
Thanks by advance !
You can find header/footer of rml report in res_company_view.xml file server side.
The file path is : server/openerp/addons/base/res/res_company_view.xml
And the value of this header footer set default from:
server/openerp/addons/base/res/res_company.py
Regards
Change openerp 7 header/footer with rml