Jeffsteez, I've been mulling over your question since you posted it, and, to be honest, I've been putting off replying because what I'm about to say, you're probably not going to like hearing.
Great thin and light pizza and good naan cannot
be baked in a 250 C with a feeble grill and without any oven mods. 250 C, by itself is not the end of the world. With a conductive enough material, such as steel, you can achieve a half decent bake time from a perspective of the hearth. As you're aware, though, if you use a material with high heat transfer/conductivity, the bottom will be done long before the top. If you use a material with lower conductivity in order to accommodate the anemic top heat, you're only going to extent the baking clock and trash your flatbreads. Long bakes are the kiss of death for thin and light pizza and naan.
I would say if you have any chance pulling a great pizza out of this oven, and you're certain that the grill is too weak to have any impact, then I'd go with the oven within an oven concept.
On the bottom shelf you want to place your stone hearth. Steel is inexpensive and readily available. I don't normally recommend putting stones on top of other stones, but with your peak oven temp, you need the transfer of steel, but, because it's so close to the bottom burner and you're going to need that burner on for the entire bake, I think some sort of insulating bottom stone is necessary. Right now, I'm leaning towards 3/8" steel plate on top of 1/4" quarry tiles for the hearth. These are quarry tiles:http://www.contempofloorcoverings.com/wp-content/uploads/2010/05/quarry-tile-basics.jpg
Sometimes they come in different colors, but you want dark red (darkest possible) unglazed terracotta tiles. You should have no problem finding these and they should be dirt cheap.
That's the hearth. For the ceiling... position another shelf from 3" to 4" above the hearth (preferably 3") and cover it with a layer of quarry tiles. If they quarry tiles can't fill the entire shelf, cover any gaps with aluminum foil. The goal is to create a false ceiling that completely isolates the bottom from the top. For the hearth, 1/4" quarry tiles would be best, but, for the ceiling, if you can use 1/2" tiles, that would be best, although 1/4" won't be the end of the world.
Now, this next part is where it gets a tiny bit tricky. The oven's thermostat is located near the ceiling and, by isolating the bottom, you're cutting the thermostat out of the equation. It's not a huge deal. As you pre-heat the oven you have to check on it and take measurements with an IR thermometer:http://www.dealextreme.com/p/digital-infrared-thermometer-with-laser-sight-32-c-380-c-26-f-716-f-29079
When it hits 250 C., you're ready to bake. Launch the pizza, make sure the burner is cranked to high and keep the door closed for the first few minutes. Now, just to be clear about something... as the pizza is baking and the bottom burner is (hopefully) pumping out lots of BTUs, the lower compartment you've created will rise a bit above 250 to somewhere around 275. This is perfect safe for your oven and is most likely in the temps it normally operates at. Theoretically, with the thermostat out of the equation, you could safely take the oven a bit higher, but I'm aware that it's not your oven, so it's better to play it extra safe. Bottom line, don't turn the oven on to pre-heat and then go take a nap. You won't burn down the house, but you might damage the oven.
This entire setup should be extremely affordable and available. I can't guarantee you that it will bake pizza and naan in 4 minutes or less, bottom and
top, but, with your setup, it's your only hope.