@Mame,
The approach you describe depends a lot upon how fast your oven loses heat, and that will vary a lot depending upon ambient temperature and the design of the oven. The Kitchen Aid Architect series stoves, for instance, have very large vents that allow heat to escape quickly so they would not work well with this approach. It would be better to follow your recipe at 500, but then dial it down to 250 for 2 hours to ensure the intended minimum temperature.
Methods similar to this are standard practice in the culinary arts, for instance grilling a steak over a very hot fire then placing it in the oven to reach a uniform dull red inside instead of a bloody purple.