Thanks for your reply.
I think that I am going to go with <aside> with a HTML5 DocType.
I think that visually, they will look OK in the site. I will try to ensure that any phrase used in a call out will stand on its own as an idea or thought.
The most irritating thing I can imagine is that the duplicate sentence or phrase will turn up either a little before or after the phrase as it appears in the text.
I think if it was on a static site, I'd consider putting the content at the end of the passage and style it individually. In this case I am putting it on a Wordpress site and I want to make it available to the site owner to use with shortcodes. So I have to add a generic style which will be safe enough for any context.