Since OpenCover supports Branch Coverage (you can find this in the xml report: <BranchPoints>), would it be possible to extend the ReportGenerator to do as well?

Of course it is possible to extend ReportGenerator. But does it make sense?

Is this piece of XML really useful?

<BranchPoints>
  <BranchPoint vc="1" uspid="27" ordinal="0" offset="10" path="0" />
  <BranchPoint vc="0" uspid="28" ordinal="1" offset="10" path="1" />
</BranchPoints>

What should the reports display?

I think this is only useful for OpenCover, since it calculates the branch coverage based on this information. I probably can't map this information to the source code.