Which end of the Hadean are you going for? There was the early Hadean with molten surface and a rock vapor atmosphere, and there was the late Hadean with liquid water oceans at a balmy 230C with an atmosphere at around 27kPa.
Early Hadean requires lots of surface heating so the bombardment and radiation ideas are the way to go. A more subtle method of creating the late Hadean would be interior heating.
A Kinder, Gentler Planet Cooker
Alexander already suggested infusing the earth with radioactive elements. If you could deliver a large dose to the core, conduction and convection would take care of the rest. A large critical mass of dense fissile material (and what fissile material isn't dense?) placed on the Earth's surface would naturally find its way to the core with local scarring to the crust and mantle.
Another way to heat from the inside out would be akin to a microwave, or to a Bubblegram. Normal radiation wouldn't make it though the atmosphere very effectively, but bombardment by a focused neutrino emitter could warm the core via Cherenkov radiation and trigger nuclear changes within heavy elements, infusing radioactive material directly into the core. This would require some unknown technologies, like aimable neutrino emitters, and probably neutrino reflectors to get enough interactions.
So, a large high energy neutrino emitter bombarding the planet from a 60 degree spherical cone converging on the core, so that there is relatively low neutrino density on the surface but a small area of extreme bombardment in the core, combined with a reflector reflecting back the same 60 degree cone to increase interactions would bias heat toward the core, gently warming the planet from the inside.