How can I add plugin for class_block.php

[galerio]

I'm trying to add Gravatar images to forum's sidebar widget for my plugin "Gravatar as default avatar". Unfortunatly there is no hook in \includes\calss_block.php and I need to manually modify the code.

The plugin is really simple, if the avatar url is empty, then it calls the gravatar url.

Is there a way to do it via plugin and not with a manual edit? (I'm not asking the code, I just ask if there is a templete hook to use).