Most people think that if the contract clearly states that the cable and brace is intended to reduce risk of failure and not eliminate it then you are sufficiently covered. ...any tree can fail and the only way to eliminate the risk of damage is to remove the tree entire... and maybe have the client initial the statement along with signing the agreement.
IMO, if you have them sign a document that they are assuming liability for a system you design, then you are not showing a high enough level of confidance in it.
The only time I would think of doing that is if removal was the better choice IMO and they refused it and wanted to go with a C&B system, but then having that in the agreement should still cover you in the case of failure that was not due to your actions, such as placed too low, or cables breaking....