It looks like the Reading List plugin (
https://www.mobileread.com/forums/sho...d.php?t=134856) does custom columns, you could see custom column code from there to see how to add/read them.
As for the HTTP RPC, I'm not aware of any plugin that listens and processes RPC events but you could probably look though calibre's code (Kovid or anyone more familiar with the calibre code could point you at the right place) to see how to best handle listening and responding to requests, or point out any reason why that may not be such a good idea.