I haven't try but I think it should work
Find function a_selection($post) in qa-theme-base.php
in that function find below
if (isset($post['select_text']))
$this->output('<DIV CLASS="qa-a-selected-text">'.@$post['select_text'].'</DIV>');
Here you can add any text after Best answer text. Make sure now condition doesn't wrapped with brace as it is single line but if you add multiple line than use brace or endif; So your code would be something below
if (isset($post['select_text'])):
$this->output('<DIV CLASS="qa-a-selected-text">'.@$post['select_text'].'</DIV>');
$this->output('your html or simple text here');
endif;
EDIT:
Sorry forgot to mention that use this in advance theme (Snow or other) don't directly modify in core file. Or create a plugin using layer