Aha! While waiting for replies, I kept digging and eventually pieced together enough information from here and there to solve this puzzle. When I finish my current project, perhaps I'll write a proper tutorial, but for now, here's a quick overview of what I did, for whatever help it may be to others. BTW, these steps may need to be streamlined along the way.
Key concepts:
1) Create a textbox the size you want in blank space, in your working document or a new one. Enter and format text as usual, using styles if you're doing several, or directly if it's just this one. Columns and Text Distances under the Text options are the place to add padding inside the box.
2) Add fill or whatever using Properties for the text box. I know there are several ways to control outer padding, but the one I'm using right now is a white line as a box border, the width of the desired padding.
3) Text boxes are shapes. Shapes can be converted to text containers. Okay! Text wrapping is controlled for text boxes as for all shapes, in the Shape section of Properties.
4) Move that box into final position within the text. Drag or cut and paste.
Not so difficult after all, and now the pieces of the puzzle are more or less assembled in one place.