The canvas platform has a feature where you can select a shape from the palette and drop it on to a shape on your canvas and it will replace that shape. This also works in containers.
This sort of works in Text Flow, with some shapes like the highlighting ones. You select a word in the TF tool, drag a highlight shape on to it.... And it puts that shape in the position of the word, pushes the word down on to the next line, but is grouped with the word. Unfortunately this also pushes the shape/word object out of the reflow. It feels like this should just work or that it almost works or I'm doing it wrong.
This would be amazing if it just worked. Select one or more words. Drop a highlight on to it. It places the line under, through, or the circle around the selected words, becoming a group with the words and still participating in any reflow. It almost works...