I have noticed that Dorico’s bass clarinet transposition is partly incorrect. Bass clarinet transposes down a major 9th ie a written middle C in treble clef sounds a B flat one octave and a tone below middle C. Dorico notates this correctly. However, when a bass clarinet part is written in bass clef (composers like Shostakovich did this, and many bass clarinettists prefer a bass clef part, as it keeps more of the music on the stave) it transposes down a major 2nd. So, the lowest written note is C below the bass stave (lowest note on a low C bass clarinet), which sounds a B flat a tone lower. Put another way, the note that is written as a middle C in a treble clef bass clarinet part, should be written one octave lower in a bass clef bass clarinet part (2nd space bass stave). Dorico seems to have got this wrong and does not account for the two different ways of writing a bass clarinet part. Any chance of a fix for this? Currently, any bass clef music I write for bass clarinet shows most of the notes in red as out of range … which they are not!
There are three Bb Bass Clarinets in the instrument picker, to cover the various different ways of notating Bass Clarinet music.
I’m guessing you need the B Flat Bass Clef Full Score instrument.
Unfortunately, that one will only put the music in bass clef in an untransposed score—none of the available instruments has the behaviour @Mike_Halliday wants. I’ve recently encountered this in Wagner too.
EDIT: In Setup mode, right-click on your Score layout and choose Clef and Transposition Overrides. Choose the bass clarinet from the list, then set it like this:
Repeat for the part layout as needed.
If that’s the case, can’t you stick an explicit bass clef at the start of the flow, then set its Octave Shift property to 1?
That would work too, depends which way feels less hacky to you, I suppose. 
BTW, if I remember correctly from my score playing exercises, there are both ways of how BCl can read bass clef.
Stravinsky I think uses the other one, while Wagner uses both, IIRC.
The whole set up is quite bizarre here; when you add bass clarinet BC it gives you a treble clef score. The second option gives you a bass clef score but no automatic transposition. I’ll have a look at the workarounds suggested and see what happens. Whatever I do the transposed part is always by a ninth, but in a bass clef part it should be by a 2nd.
Thanks for help with transposition overrides … I have managed to make the music look right now as well as transpose correctly. Still seems a bit strange that this requires a workaround, though.