A pressure washer only works when the jet from the nozzle hits the crud. So just hooking it up to the drain plug won't help much. Not much better than just a garden hose, anyway.
If it's got lots of stuff in there, it might be better to pull the core plugs and knock whatever's accessible from those openings loose and flush it out first. Evaporust is expensive, and too much rust and crud will exhaust it prematurely.
It definitely works better warm than cold, so filling the system and running the engine would help. Of course, the core plugs would have to be reinstalled to do that.
