If you have access to a sound set with a contr’alto flute sound, you should be able to change the name of a similar player and assign it your sound (along with an expression map that does the appropriate transposition for you).
If the Dorico Team adds a contr’alto flute but neither HALion nor NotePerformer supports it, that will be a Pyrrhic victory.
Of course the Team has often mentioned their intention at some point to allow users to create custom instruments. Although that will not necessarily supply the sound, it will perhaps get you halfway there.
If not, you’ll need to edit them to get the correct results. I wasn’t exactly sure how you wanted to view them in concert pitch with octave displacements, but the transposed pitch should display correctly.
Unzip the above file, then add Flutes.doricolib to your DefaultLibraryAdditions folder. In Windows this lives at Users\yournamehere\AppData\Roaming\Steinberg\Dorico 4\DefaultLibraryAdditions. If the folder doesn’t exist, go ahead and create it. After restarting Dorico you should see the new Custom option in Setup.
Obviously you can edit the file to add whatever instruments you want. You can copy, paste, and edit any entries from my file or the factory instruments.xml file as needed. You can obviously edit the name to be “Contr’alto” too if you want. Hope that helps!
Thank you for your answer Derrek. Actually NotePerformer supports that, at least in that way that it down transpose the Bass Flute. I use the NotePerformers Bass Flute for Contrabass Flute parts in Dorico and it works quite nice although a bit sluggish.