I would have thought a muntjac lacks enough fat to make it a decent spit-roasting candidate. That said, I think you could stuff it with a stuffing containing a fair amount of fat and moisture (herbs, breadcrumbs, fatty bacon, something like that), sew it it, and truss up the legs so that they're up against the body, as you do a chicken. You'd then be cooking a more homogeneous mass that would be to some degree self-basting.