Update entry.body from Plugin
There is no event or hook that I'm aware of that can intercept the template rendering process directly. You would likely need to instigate this from within your template. I would suggest perhaps using a plugin variable either directly, or as a wrapper for your plugin service method (if you need to do something more complicated). 

Something like the following:

Note: This is adapted from the 'BusinessLogicTemplate' plugin which provides a great starting point for plugin development.

&lt;?php namespace Craft;

class MyPluginVariable
{

    /**
     * Whatever you want to output to a Twig tempate
     * can go into a Variable method.
     *
     * HOW TO USE IT
     * From any Twig template, call it like this:
     *
     *     {{ craft.myPlugin.exampleVariable }}
     *
     * Or, if your variable requires input from Twig:
     *
     *     {{ craft.myPlugin.exampleVariable(twigValue) }}
     *
     */
    public function exampleVariable($optional = null)
    {
        return "And away we go to the Twig template...";
    }
    public function formatBody($body)
    {
        /* process $body and return it to the template */
        return $body;
    }

}


Then in your template:

{{ craft.myPlugin.formatBody(entry.body) }}