Umm, actually 5.56 doesn't have much recoil at all. I mean, it's been almost 4 months since my last qual, but I'm pretty sure simulating that recoil is well within HPA doability.
I'm not saying that a 5.56 has a lot of recoil, but its got A LOT more than these drop in HPA systems. Look at that first video he posted.. the gun doesn't move an inch and he's not even shouldering it (which is fine, its an airsoft gun). Sure he's using a bi-pod, but watch any youtube video of an AR-15 shooting with a bi-pod and compare the two. Maybe you could get more recoil with a blow-back version, but; that certainly wouldn't be a 'drop in' and would require a lot more modifications (I think it's safe to assume).. and not to mention wasting gas you could be using to shoot BBs at people.
However, the video is exciting because it shows us progress! I would love to see a video of you shooting it through a chronograph. Maybe RPS and FPS? Also, I'm not sure if the system is 'there' yet, but a video putting the system to its limits (safely of course). Seeing how many FPS or RPS you can get out of it.
Also, I'm only going to say this again cos he posted a video and now we can see the system. I know it doesn't bother everyone but it's just my opinion. The gas line cover is huge.