[EDIT] Added fiddle. Cleaned up some code. Updated snippet below. Added comments.
[ORIGINAL] I don't believe that this is possible with css. However, I figured a solution could be possible with JavaScript. So, I tried it, and it worked! While this might not be the ultimate solution, consider the following:
<div class="jsCenterOnPhrase">
The quick brown fox jumped over the lazy dog.
The quick brown fox jumped over the <div class="jsCenteredPhrase">lazy dog.</div>
The quick brown fox jumped over the lazy dog.
The quick brown fox jumped over the lazy dog.
The quick brown fox jumped over the lazy dog.
</div>