Here's another way to go. Put this tag in the templates that should get the button, near the top of the body section (above any *content tag):
Put the button HTML code in a chunk called "MyButton".
Put this placeholder in the content where the button should appear:
Create a snippet called ShowButton with this code:
<?php
$chunk = $modx->getChunk('MyButton');
$modx->setPlaceholder('MyButton', $chunk);
return '';
