Hi,
Your idea seems correct to me, to be honest, the code seems to be okay. However I am not sure where the ‘rh_no_thumb_url’ custom filter is executed, it might be out of the loop. It is also possible that the post data from the search is no longer available at that point, and then the function is not able to return it.
If you want, you can add temporary FTP details, and I can try to debug through the code, to see what exactly is happening.