The only product we have  that can do such a thing is this model 

Unfortunately, what you are asking is generally not possible with a standard HDMI-to-AV converter.

The reason is that HDMI does not carry closed captioning data in a format that can be passed through to an analog TV as separate Line 21 closed caption data. In most cases, when a decoder, DVD player, or set-top box has Closed Captioning enabled, it decodes the caption data internally and overlays (or "burns") the captions onto the video image before sending it out over HDMI. At that point, the captions are no longer separate CC data—they have become part of the video itself.

This is why most consumer HDMI-to-AV converters do not pass Closed Captioning. There is simply no separate analog CC data available for them to convert.

The only practical way to preserve analog Closed Captioning would be to use a video source or decoder that provides a native CVBS (composite video) output with Line 21 Closed Captioning embedded in the analog video signal. In that case, the analog TV can decode and display the captions normally.

If your current decoder only provides HDMI output, there is generally no way to recreate the original analog Closed Captioning data from the HDMI signal. In that situation, you would need a different decoder or video source that supports composite video output with Closed Captioning.