I've never seen this done, but was wondering if it's possibe. If you get a small enough gas block on a mid-length gas system, can you put a rifle-length free float rail on it?
I have a couple AR's with the set up you are looking for, I have used JP Enterprises JPGS-8 minimized adjustable gas blocks on them. Their a bit pricey at $80.00, but they are top notch parts and fully adjustable to boot. Here's a link: http://www.jprifles.com/buy.php?item=JPGS-8