So, go ahead and check out a release branch, set the "Official" build flag yourself, wait anywhere from 2-8 hours for the binaries to get built, and verify it against the bits we ship.
I went through it and there's still some ugly hackery involved on Windows. These are technical issues (e.g. how ffmpeg is compiled via MinGW), but they make it complicated to generate a non-branded, psuedo-official build. That said, all the pieces you need (minus the closed-source plugins) appear to be in the repository and can be built and compared against an official release. It could definitely be made easier, but would require some non-trivial engineering to do so.
So, go ahead and check out a release branch, set the "Official" build flag yourself, wait anywhere from 2-8 hours for the binaries to get built, and verify it against the bits we ship.