I don't really understand what the goal is here, you would use a Request DTO to define which Parameters a Service Accepts and Response DTO to define what your Service returns, the explicit purpose of the Request/Response DTOs is to define your Services Contract, i.e. the most important contract in your System, whose well-defined interface is used to encapsulate your systems capabilities and is what all consumers of your APIs binds to.
The C# POCO used to define your Request/Response DTO classes should be considered as a DSL for defining the inputs/outputs of your API, trying to collapse and merge their explicit intent of your APIs into multi competing Types with custom attributes is self-defeating, it adds unnecessary confusion, blurs its explicit definition which invalidates the primary purpose of having service contracts which is what all metadata services look at for documenting your API and generating the typed bindings in different supported languages.
So the approach and desired goal for using custom attributes for controlling serialization behavior so you can reuse the same types in different contracts is highly discouraged, but should you wish to continue with this approach you can refer to this answer for different ways to Ignore properties in ServiceStack.Text, specifically ShouldSerailize() API which will allow you to dynamically specify which fields ServiceStack.Text should serialize, if you intend on implementing a convention you can delegate the implementation to a custom extension method, e.g:
class MyRequest
{
public bool? ShouldSerialize(string fieldName) =>
MyUtils.ShouldSerialize(GetType(),fieldName);
}
Other than the linked answer the only other opportunity to manipulate serialization is potentially to use the built-in AutoMapping utils for selecting which properties should be copied over and the Object Dictionary APIs for converting C# Types into an Object Dictionary and manipulate it that way, then can dehydrate it back into the C# type after applying your conventions.
