You might try inserting a graphic image of a vertical bar as a floated image. You can insert the image tag right at the beginning of the sentence that you want to mark.
Code:
/*in the CSS*/
.barleft {float: left;
margin: 0 3px 0 -.5em;
width: 3px;
height: 5em;
max-width: 10px;
}
/*in the HTML*/
<p>blah blah blah blah <img alt="" class="barleft" src="../Images/bar.png"/>NEW BLAH NEW BLAH NEW BLAH. old blah blah blah.</p>
The negative left margin would make it display out in the margin, rather than flush with the text.
Of course, for different lengths of sentence you would have to style for different image heights, and it might not be all that accurate (although using ems for height might help).
If it is only for personal reference, you might not be concerned about height of the bar. Or, maybe instead you could simply have a small image similar to a bullet to indicate that "here be changes."
Maybe different font-faces for the different edits might be a way to go if you need it to display on a monochrome reader and can't use colored text.