Unfortunately, I don't have any pictures with mine on a 22mm Nato strap. The lug width of the BM6400 is as you stated 21mm. Personally, I like the slightly wider 22mm strap, it looks better proportioned to this watch. The 20mm just looks too skinny to me.
On the BM6400 the 22mm is just ever so slightly "squished", but to tell you the truth it's not noticeable. Now, being narrower the 20mm exposes some of the springbar which just kind of bugs me.
I really recommend that you get one and see how you like it. Try a 22mm, it'll work fine. Just don't over-think this and obsess about 1mm. I mean, it's just a watch strap right?