Is it possible to get the position or the reference (paragraph number) of a book's selectedText?

Also, is it possible to get the text of the paragraph that the selectedText is located within?

Also, i'd like to be able to highlight the selectedText in a different color, and have that persist while the book is open. My assumption is that i would have to insert a <span></span> around the selectedText. Would this be something I'd do via Qt or javascript?

An indirect answer to your question...

The viewer has a "Reference Mode" that can be turned on with the icon that looks like a tin of paint. When it is on and you put the cursor over a paragraph, it highlights the text and then displays a hint with the paragraph number. The code behind that should answer part of your question.

Alll these things are possible using javascript, just google for how to do them and you will find plenty of hints. And you can change selection color using a special selection CSS selector, again googling will tell you how.