You can always just add a new page to the domain to be used for #VE.
It's there a method to directly assign physical pages to guest from dom0? Using xc_map_foreign_address just like libvmi?
Please don't top-post on xen-devel.
You could share a page from dom0 but I think what you want to do is increase the reservation of the domain and then map it into so it can be used for #VE. The functions for this are xc_domain_increase_reservation_exact and xc_domain_populate_physmap_exact.