@mariosipad
Thank you very much for your response!
- I was afraid of that. COPS' code looks more complicated than chaley's Calibre-PHP server.

- Yes, pretty much all custom columns. I'm guessing the handling is different. I've looked under the proverbial hood (metadata.db) with SQLite Browser and as far as I can tell, the tag-like custom columns work the same as tags. I'm guessing it's because one has to explicitly specify in config_local.php which custom columns to display.
- Looks like more coding to display more tags.

- Hmm, weird then. Iirc, not even my standard Tags column had clickable links in book detail view.
- The comments thing would probably be easy to change.