The SC2000 reflector sight for the M249 doesn't have a secondary optics mode for using the weapon's iron sight. However, the 4x20 scope, which is mounted on an identical riser, does.
The reflector sight should allow switching to iron sights, since they're unobstructed, just like the 4x20. There's no difference in the mount which would prevent it.
Sorry, you've misunderstood what this is about. I'm not talking about additional sights on the optic itself.
On the CSLA M249, optics (both the 4x20 and the reflector sight) are mounted on a riser bracket, which sits on top of the built-in iron sights. The bracket doesn't actually block the iron sights, and with the 4x20, you can switch back to using them, giving this view:
The reflector sight uses exactly the same bracket. So the iron sights are also unblocked, and logically should also be usable. It's hard to get a good angle in first-person, but you can kind of see here that the iron sights are unobstructed, exactly the same as with the 4x20 scope. You just can't switch to them.
Here's an MSPaint diagram that will hopefully explain better.
