As long as you have filled the radiator prior to startup, that's about all you can do, unless you want to keep topping it off as the engine runs.
You could "fill" the block some through the thermostat hole, if you wanted to pull the 'stat and go that far, but you'd only get a little bit in there, but hey, if that gives you peace of mind, go ahead, it certainly won't hurt. Might put in a new thermostat and gasket while you're at it.
Even if the block is completely drained, when you fill the radiator, some of that goes into the block. You can get a whole gallon of coolant/water in there initially. Once you start the engine, the water pump instantly gets coolant circulating, even without the thermostat open to let it circulate through everything.
You could technically run an engine with NO coolant in it up to operating temp, then shut it down before it starts to heat up/overheat. The real damage is done when things go beyond operating temp. Of course, having water/coolant in there helps transfer heat from the pistons and walls to the rest of the block, so it's not really a good idea to run it dry. But as far as a drain and refill, there's no need to do anything more than the regular fill/air purge procedure.
To refill, just fill the radiator once you're sure everything is sealed up, then put the radiator cap on, but only to the first click (loose), not tight. This lets air escape. Start the engine, and let it idle until the temp gauge needle gets to the middle of the gauge, and goes a little to the right, then falls back to the left. This means the thermostat has opened, and coolant is circulating throughout the whole system. After a few seconds, shut it down, let it cool a few hours. Come back once it's cold and refill the radiator again, plus top off the overflow reservoir. Start it again, with the radiator cap still on just the first click, let it get up to operating temp, make sure the thermostat opens, let it run for a few more seconds, shut it down, let it cool. This time, top off the radiator, top off the overflow reservior, put the radiator cap on tight and you should be done.
Of course, it's always good to keep an eye on it and check the level of the overflow reservoir to see if it's going down, which would mean there's a leak in the system somewhere.
