Has anyone seen a hk style rear sight for a flat-top?
Yes. But they are as scarce as hen's teeth.
And when they do show up, they are very expensive. > $300 a set
There is a member here with a set mounted on one of his rigs, and he claims they are inferior to other popular BUIS options
They are used on the new HK 416/417 systems. (But essentially the same one used on many HK weapon systems - MP5 for example)