Doesn't sound like too hot of an idea. I guess if you get a true monolithic upper you can do this if you want. But I have KAC FF rails on a couple of rifles, and I can tell you that as nice and solid as those are, I'd never ever mount sights on them. I think there are also cantilever mounts that might work. But putting it on the actual forearm is a bad idea.
In the army, we'd mount lasers on the rails, FF or not. Nowhere else to put them. They lost zero rather quickly, no good groups.
I also knew a few guys that tried to put their M68's on the rail. That was a colossal fail. They missed every target past 50-100m.
Finally, if you get an EoTech, those have such a big window and they actually work better up close with both eyes open. If you are thinking of a scope mounted foward, well, I can't help you there. Some folks think it is a good idea, and at least one training school teaches it. But if you need to track fast, I still think the EoTech is better. And they make one now with a BDC for a .308 and you can get a magnifier for it too and now you have the best of three worlds!
However, if you get an AR10 and it is a good shooter, you can be seriously hindered by poor optics. For the price and the quality, a TA01B, the .308 ACOG with stadia, is hard to beat. I love mine. Not a scout mount, but then again, the only scouts I ever knew used ACOG's and M68's.