I was interested to do this but it doesn't work. If you try to change to viola clef it only changes the beginning and leaves the subsequent tenor and bass clef intact. So how about a proper transcription for viola?
What if you copy a part of the score after the first clef change and paste it to a blank score? I think it will transpose this part too, then. (Because in this file there won't be any clef changes.) And if there are several clef changes, you could do this several times and eventually make a single score off a few files (again, by copy-paste).
Thank you Mailila for the suggestion. Unfortunately that is not how the program and files work. We will have to wait for Sibelius to update their software to fix this issue or to produce the transpositions ourselves.
Virtual Sheet Music, Inc.