I found it hard to work out an answer: a snapshot database can't be a shared scalable database - so presumably if I could take a snapshot of a shared scalable database the snapshot wouldn't have that property; but that's not a problem, a snapshot database doesn't have to have all the same properties as what it's a snapshot of (it has allow_snapshot_isolation ON even if the original database has it OFF) so I can't draw any conclusion from that.
I couldn't find anything in BoL that told me the answer (not even in the reference supplied in the answer).
So I wondered about what's the point of such a snapshot. The shared scalable database is already a read-only copy of the original database at a particular point in time, so maybe there's no point. I decided that I couldn't see any other reasons for answering one way or the other, so my answer had to be no, you can't do it. Luckily that's the right answer. Of course I still don't know whether to believe it, because it doesn't appear to be documented.