I don't know anyone that makes a flip down rear diopter except HK. There are a few companies making "HK style" flip down sights, like Troy and KAC, however those are just M16 style sights with a ghost ring instead of blades. The elevation on these sights is still meant to be done with the front post (which defeats the purpose of the front sight always being in the center of the circle, but apparently that is lost on Troy and KAC) , so if you leave the stock HK front iron, you will have no way to adjust elevation. You have to replace both front and rear if you replace the irons.
Depending on your Acog mount, you may want to go with standard height irons instead of the HK/Micro height, so you can cowitness through your Acog. . .. you know, in case the batteries go bad (that was a joke).
-W