What is your proposed syntax?
The format would be the same as supported by java.lang.String.format() (see javadocs).
Ideally, you would just use java.lang.String.format() to format the message. The only tricky part is handling el-style expressions, perhaps by first expanding the el-style expressions, then passing the result to String.format().
Looks good - create a JIRA reuqest and attach your patch!
Bear in mind we need to maintain backward compatibility - this should be an extra.