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The OPDS client chooses what columns to show/search, not calibre. I don't know what client you are using, but ask whomever wrote it. All the custom columns are available to an OPDS client.
Alternatively use the native calibre content server. It knows about custom columns. Even the very limited /mobile version (no javascript) offered by calibre knows about custom columns. If you are using the OPDS server then the normal and /mobile versions are also available.
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