Place a custom block inside admin area
You can just go to "Structure > block layout" from the admin bar. This will default to your current default theme but along the top of that page it will list out all other active themes and "Seven" will be one of them

Click that and the block listing will now be for the Seven theme and you can add/remove blocks like you can normally except it will now be for the Seven theme

You wouldn't be able to place a block midway through the form but you could add a block above/below the form