Custom rendering extensions not working in SSRS Report Designer (rsreportdesigner.config)

Question

My custom rendering extension is working in Report Builder (in RSReportServer.config), but not in Report Designer (in RSReportDesigner.config):  instead of adding a "TXT" export option, it's just adding a duplicate "CSV" option.
SUMMARY: 
Is there a way to get these features working, with Report Designer?
DETAILS:
We are running SSRS (Reporting Services) under SQL Server 2008 R2.
Here's my rendering section:
    <Render>
      <Extension Name="XML" Type="Microsoft.ReportingServices.Rendering.DataRenderer.XmlDataReport,Microsoft.ReportingServices.DataRendering" />
      <Extension Name="CSV" Type="Microsoft.ReportingServices.Rendering.DataRenderer.CsvReport,Microsoft.ReportingServices.DataRendering" />
      <!-- TXT extension with help from:  http://social.msdn.microsoft.com/Forums/sqlserver/en-US/d79845a8-17fb-4ec6-b121-2c40cf466d73/how-do-i-add-a-pipe-delimited-option-in-ssrs-2008-report-manager?forum=sqlreportingservices -->
      <Extension Name="TXT" Type="Microsoft.ReportingServices.Rendering.DataRenderer.CsvReport,Microsoft.ReportingServices.DataRendering">
        <OverrideNames>
            <Name Language="en-US">TXT(ASCII,NoColHds)</Name>
        </OverrideNames>
        <Configuration>
             <DeviceInfo>
                 <FileExtension>txt</FileExtension>
                <FieldDelimiter>,</FieldDelimiter>
                <Encoding>ASCII</Encoding>
                <NoHeader>true</NoHeader>
            </DeviceInfo>
        </Configuration>
      </Extension>
      <Extension Name="IMAGE" Type="Microsoft.ReportingServices.Rendering.ImageRenderer.ImageRenderer,Microsoft.ReportingServices.ImageRendering" />
      <Extension Name="PDF" Type="Microsoft.ReportingServices.Rendering.ImageRenderer.PDFRenderer,Microsoft.ReportingServices.ImageRendering" />
      <Extension Name="HTML4.0" Type="Microsoft.ReportingServices.Rendering.HtmlRenderer.Html40RenderingExtension,Microsoft.ReportingServices.HtmlRendering" Visible="false" />
      <Extension Name="MHTML" Type="Microsoft.ReportingServices.Rendering.HtmlRenderer.MHtmlRenderingExtension,Microsoft.ReportingServices.HtmlRendering" />
      <Extension Name="RPL" Type="Microsoft.ReportingServices.Rendering.RPLRendering.RPLRenderer,Microsoft.ReportingServices.RPLRendering" Visible="false" />
      <Extension Name="EXCEL" Type="Microsoft.ReportingServices.Rendering.ExcelRenderer.ExcelRenderer,Microsoft.ReportingServices.ExcelRendering" />
      <Extension Name="WORD" Type="Microsoft.ReportingServices.Rendering.WordRenderer.WordDocumentRenderer,Microsoft.ReportingServices.WordRendering" />
    </Render>
When I add the above "TXT" section to RSReportServer.config, then Report Builder (and production) both show an export option "TXT(ASCII,NoColHds)". 
But when I add this "TXT" section to RSReportDesigner.config, and then (in Report Designer / BIDS) attempt to export from a "Preview" of the report, the export drop-down does not show a "CSV" option followed by a "TXT"
option, but instead shows the "CSV" option twice.
This simply limits testing of the export option, to Report Builder... or requires any reports be published from Report Designer before they can be tested with
this export option.
(FYI, why this export option:  compliance in our industry requires sending data to government agencies, in text files, with fixed-length columns, and ASCII encoding.  Also, we're attempting to give the production of these files to the *users* managing
communication with those agencies... so we're putting them into SSRS.
I worked around the fixed length columns (and no delimiters -- no commas), by writing a version of the report where all string columns are padded, and all columns are concatenated, to form ONE LONG COLUMN... but Reporting Services' CSV export format produces
a Unicode file (UTF-8, which the agency rejected, because it had a leading "", or "EF BB BF" in hex), whereas the agency requires an ASCII file.)

Answer 1

Hi Doug_atMidway,
According to your description, you want to enable your custom render extension. Right?
In Reporting Services, if you want to deploy the custom extension, you just need to add the extension into rsreportserver.config file.  The
RSReportDesigner.config file stores settings about the rendering extensions available to Report Designer. Since you still use the csv rendering extension in your assembly, we don't need to do any modification in rsreportdesigner.config file. Pleaes
refer to the links below:
Thanks for attempting to help, Simon.  
As my question states, I've *already done* both the above:  changed (1) rsreportserver.config and (2) rsreportdesigner.config.  I added the same code, shown above, to both files.  I did that so I could see the new "txt" extension
both (1) when exporting in production and Report Builder, and (2) when exporting in Report Designer's "preview".  
The change in (2) is not working:  I do not see the "TXT" extension in Report Designer, when I try to export from a preview.  Instead, Report Designer shows the CSV extension repeated.
Thanks for the docs.  I consulted them (well, I consulted the EQUIVALENT pages, for SQL Server 2008 R2), when creating my block of code above.
Do you see anything to correct, in my code?
Are the features I'm using actually working, with rsreportdesigner.config?
Thanks again, 
-- Doug