Hmm i don’t think you can make this effect just using 3d stroke.
U can use 3dstroke as a matte, to reveal your flag image, but you wont get your flag layer in real 3d space. And in this case, you don’t need plugin to do this.
the real way to do this is in a 3d software (3ds, maya, xsi,…)
maybe with a plugin like 3d invigorator? i’m not really friendly whit this one. but the thing is you can’t do this with 3dstroke only!