Hello, provided I am not an expert and therefore I might even misunderstood your question, I believe I managed to achieve that by doing the following :
(it's all done in the XML file)

First, you probably added a named connector like this one :

   <CONNECTOR
      NAME="REJECT"
      CTYPE="FLOW"
      MAX_INPUT="0" MAX_OUTPUT="1" LINE_STYLE="2"
      COLOR="086438"
      BASE_SCHEMA="FLOW"
    />

Then in the parameters you need a schema type :

<PARAMETER NAME="SCHEMA_REJECT" FIELD="SCHEMA_TYPE" REQUIRED="true" NUM_ROW="1" CONTEXT="REJECT">
     <TABLE READONLY="true">
      <COLUMN NAME="REJECT_ID" TYPE="id_Integer" READONLY="true" CUSTOM="true"/>
      <COLUMN NAME="WHATEVER" TYPE="id_String" READONLY="false" CUSTOM="true"/>
     </TABLE>
    </PARAMETER>

Note that this schema will not be editable from the component parameters.
The key is to set the CONTEXT="REJECT" in the parameter, where the value of the context is the same as the name of the connector.

Hope it helps.
By chance, do you happen to know the answer to this : http://www.talendforge.org/forum/viewtopic.php?id=14865   

Francesco