It’s not possible. Even if it would be problem is that content of request attributes is limited to couple hounded of chars. Even if body is Jason payload it may be to little. It may produce overhead as well. This is why we don’t have such option.
The only way is to define the request attribute based on method parameters (Java/NET/PHP). It requires the knowledge of your code and it's not guaranteed you will find the right method as it depends on the application code structure.