How is this set up?
There's nothing to it, really. Bitmap is tiled 10 times, so to match this i set tiles count to 10 by 10 (note that tiling in coordinates rollout needs to be left at default in order to get proper randomisation), then it's up to you how you want it to mix it with bitmap, in this case i used Corona mix in softlight blend mode, but you can use native mix, composite, or even RGB multiply, mask and any combination of them.
Yeah, thought about this as well, but you need the bitmap to be bigger than what you'll show because this one will tile as well. Might work depending on the case, but it's probably really easy to setup.
The bitmap is tiling the same number of times as the tiles map in this case, but it's not crucial, since the bitmap is seamless so you don't have to match their tiling. One can be tiled 5 times and the other 7 times and you won't notice anything strange.