The problem that you are having is that you have to override everything in the particular t() function, not just part of it.

So finding the relavent line in core was extremely important... so that work you did was not for naught. The line you found from core is as follows:

'title' => t('Read more<span class="element-invisible"> about @title</span>', array('@title' => $node_title_stripped)),

The mistake you were making with the string overrides module is that the part you have to override has to be all the text in the first argument of the t() function. So in this case, you need specify the following as the text you want to replace:�

Because most of the time I have to find the exact string inside Drupal core or a contrib module in order to get the String Overrides module to work, I instead never use that module but instead use Drupal core's built in way to do this via editing the settings.php file (that is NOT hacking core).

# $conf['locale_custom_strings_en'][''] = array(

# � 'forum' � � �=> 'Discussion board',

# � '@count min' => '@count minutes',

# );

Change it to this, uncommenting the $conf variable and then adding your own line, following the examples given to you there:

$conf['locale_custom_strings_en'][''] = array(
# � 'forum' � � �=> 'Discussion board',
# � '@count min' => '@count minutes',
� � 'Read more<span class="element-invisible"> about @title</span>' => 'See full article',
�);

p.s. I don't have a clue what that�<span class="element-invisible"> about @title</span> is all about. That is new with D7. But it's not just this case that more than just the visible string is added to a t() function. You almost always have to find the original t() function and see what's there before you can successfully override it.