I have
posted the first build (245M) of Calibre2opds that has some support for Custom Columns. In this build you can request Custom column values to be displayed under Book Details.
As this is the first build that supports Custom Columns there may well be some initial quirks. The list of types is initially limited until I get around to adding missing types - you will get an error message if you try and use a type not yet supported. I would welcome any feedback.