I think this could by well done by two simple interceptors:

@InterceptorBindingType
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface ConversationBegin {
  @NonBinding String id() default "";
  @NonBinging long timeout() default 0;
}

@ConversationBegin @Interceptor ConverstaionBeginInterceptor {
  @Current Conversation conversation;

  @AroundInvoke public Object conversationBegin(InvocationContext ctx){
    ConversationBegin begin = ctx.getMethod().getAnnotation(ConversationBegin.class);
    if("".equals(begin.id()){
      conversation.begin(begin.id());
    } else {
      conversation.begin();
    }
    if(begin.timeout() != 0){
      conversation.setTimeout(begin.timeout());
    }
    ctx.proceed();
  }
}

@InterceptorBindingType
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface ConversationEnd {}

@ConversationEnd @Interceptor ConverstaionEndInterceptor {
  @Current Conversation conversation;

  @AroundInvoke public Object conversationEnd(InvocationContext ctx){
    ctx.proceed();
    conversation.end();
  }
}

It would be fine to have this interceptors built in web beans standard.

It would be also fine to have some way how to have interceptors enabled by default. e.g. autoenable property of InterceptorBindingType but it could break the interceptor contract with container :(