Clearly, I was making sloppy mistakes yesterday. I fixed the issue and it was entirely my fault. In my FAKE file (F# Make) I was pointing at my tools directory instead of my test directory. Silly, silly.
So, to confirm the requirements for any future readers:
- Create a `Reporting` folder somewhere in your source.
- Copy the `NUnitExecutionReport.xslt` from SpecFlow source to `Reporting/NUnitExecutionReport/`
- Copy the `Common.xslt`, `GherkinElements.xslt`, `Languages.xml` from SpecFlow source to `Reporting/Common/`
- Run `specflow.exe "nunitexecutionreport" "<path-to-csproj-file>" "/out:SpecFlowResult.html" "/xmlTestResult:<path-to-NUnit-xml-result-file>" "/testOutput:<path-to-NUnit-txt-result-file>" "/xsltFile:<path-to-custom-xslt-file>"