Not sure if the list is still monitored ( I also added the request in the Trello board):
I am using Calibre on my notebook and sync the entire library through DS Cloud on my Synology NAS and with my mobile devices (an Android smartphone and a pad). So I always have all books stored in the cloud of the NAS any synced with all my devices.
CC shall recognize that the library is locally present and open the book from there without the need to download it or offer an UI method to access the book without downloading it. The latter probably would be easier to implement since CC knows where to download the book from and if it isn't available could fail gracefully.
Reference:
https://www.mobileread.com/forums/sh....php?p=3884091