You can add and configure a mini page for the section. In case you have a custom object and there is obviously no oob section created based on such object, you can create such section from your side, configure mini page and further hide this section from the workplace. This way you'll have the mini page for a detail created based on this custom object and section won't bother you as it's not added to any workplace, therefore not available for users from UI.
Adding mini page to the detail without creating a section based on it's object can be achieved only with a help of additional development.
We've also registered a corresponding query for our responsible R&D team to consider implementing the requested functionality in the upcoming versions of a system.