I would be curious why an LLC was not feasible but an s-corp is as it may alter the answer being sought here. With that being said, your use of the word "silent partner" begs 2 questions in my mind:
1. If you mean "silent" as in no day-to-day business operations then you will have no problem to solve for this as my colleagues already stated.
2. Alternatively, if you mean "silent" in that no one will know the identity of the S/H then a much different problem and S/H agreement and tax return K-1's pose a problem.
Perhaps you can add additional info on the two points raised to help you get more answers.