If not, I plead the developers to consider introducing this feature in the future.
AFAIK you can’t set a border on character level, but you could insert a frame that’s anchored on character level. This way, you could insert text content with a border that takes part of the text flow.
If you want the developers to introduce such a feature, you should file a request for enhancement in the LibreOffice bug tracker. Additionally, there’s a page to collect and vote for enhancement requests.
EDIT: As vmiklos wrote, there’s already such a bug report…