My first reaction was that the “sh” contraction should be used in this case. I don't think it could be mistaken for “shall” since it has an apostrophe before it. It is slang for “hush”.
For instance, in 12.5b(3) of the Instruction Manual one of the examples given is (th)' as (the) in dialect. Here the "th" contraction is used with the apostrophe, even though by itself it has the whole-word meaning "this".
What do you think?