After a few fruitless attempts at searching the 1871 census for someone by occupation and getting no results I discovered that for some reason occupations aren't recorded in the 1871 transcriptions.
This being the case, why is there an 'occupation' box on the 1871 search form?
Another email sent to FindMyPast.
Their reply is that the 'occupation field is a default search field for each census year'.
However despite the fact that the occupation box is present on the 1871 search page, and capable of being filled in, they confirm that census is
not searchable by occupation.
This is because the 1871 transcription was 'bought in' and no occupations had been transcribed.
I think at the very least there should be some sort of 'warning' about this on the 1871 page or that, as on the 'old' site, that field should be greyed out, as Dawn mentioned.
They state that they will do their own transcription of the 1871 at some time in the future.