[stumped]

i will offer just one suggestion for getting reading progress into CC. If I am the only person wanting it then I accept it will never happen, however...

..Moon reader can put that info into dropbox and CC can already connect to dropbox.
when I turn on moon reader's dropbox sync I see that moon writes to dropbox/apps/books/.moon+/cache with 2 entries per book -
booktitle - author.po and booktitle - author.an
both are 1kb files

so CC could potentially read and display what's in those,?
e.g. looking at two .po files in notepad++ I see 16% and 65.3% at the end of the strings, and i'd wager that's the read progress values
1460487918082*7@0#0:16.0%
1460487918082*20@0#0:65.3%
the opening number is the same for both books so it's maybe the moon+ theme identifier.
7@0 20@0 could be chapter and position in chapter, I usually bookmark at a chapter start. each book is uniquely identified by its file name, which was passed to moon by CC when book was first opened ?

( the .an file is gibberish in notepad, and not needed for read progress reporting )

those file pairs may also exist also in local storage, I have not looked

a later generalisation would be a preference to select from list of supported readers for reading progress reporting, and support coud be developed one reader at a time , if there is sufficient demand