This would be great

Let's not decline this so soon.

1) as you mentioned, there are numerous file formats that do not support modiying the internal metadata, so the only option for those is to use an side file. The fact that .opf exists at all (as an open spec to boot) indicates that there's a use for it

2) the comicbook community has a sideload format as well, so they see a need for it as well https://wiki.mobileread.com/wiki/CBR_CBZ#ComicBookInfo

3) there are a number of sharing communities in which modifying the original file is verboten - DC++, Torrents, etc - once you have altered the original file, you've essentially become useless. Another community I participate in actually brought this up for managing comics as well. 

Do you use something other than Ubooquity?

Can you share your startup script?

bump? any have this set up?

What did you use for your reverse proxy prefix?

Using what you posted above makes everything go to the library UI, but can't get to the admin page.

Is there a way to make ubooquity drop a process id file somewhere?