Have you tried using the underline option in the chapter title paragraph style (as opposed to paragraph rules)? It will only underline the text, it won’t extend across the whole column like a paragraph rule would, but it does export to ePub. If you have to insert the line as an image you could position a piece of text (I use 3 asterisks) with its own paragraph style, centered under the title. Then create your line (or use any kind of image, divider, or flourish you want) on the pasteboard, cut it, then in Find/Change search for *** and change it to ^c (contents of the clipboard). It will replace all instances of the 3 asterisks with your line and each line will be anchored in place under the title. If you need to move the line up or down, you can do that by adjusting spacing of the *** paragraph style. Of course you have to go through your book and insert the asterisks, but it’s a lot easier than pasting and anchoring the line one chapter at a time.
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