Thank you for providing the cool plugin.
Could you make a new configuration item for rendering custom fields in Product backlog (like in Sprint board) ?
There is a message forum:"Inherit sprints and product backlog in subprojects" in this forum, but it may be a difficult request. Therefor, I'm thinking about collecting all issues in subprojects into the root project and use a custom field instead of subprojects.
It seems to work fine, but I can't see the custom field in Product backlog. It's important information to sort items in Product backlog.