How to cite the US Federal Register in Harvard referencing style?
My approach would be to give an in-text citation as:

Federal Register (2012)

The end-of-paper reference could be given in one of two ways:

Federal Register (2012) National Forest System Land Management Planning. Federal Register Vol.77 No.68, 21161-21276. Washington D.C.

or....you could cite the online pdf version as follows:

Federal Register (2012) National Forest System Land Management Planning, Federal Register Vol.77 No.68, 21161-21276 [pdf] Washington D.C.: Forest Service. Available at: https://www.gpo.gov/fdsys/pkg/FR-2012-04-09/pdf/2012-7502.pdf [Accessed 27 June 2018].

I think it's important to remember that the purpose of a citation is to help the reader to be able to access the document in question.  In my examples above, even if there may be potential disagreement over authorship (is it the Federal Register, US Govt. or Forest Service?) or location of publication, the reader can easily find the document you are referring to (especially the second option, which takes them directly to the online pdf).