require_once() is fine, but if the include_path setting does not, well, include the path to the directory where that file is, then it won't find it. As a quick, check, just create a little test script (or stick an error_log() in an existing script) to see what the include_path is. (In fact, your require_once error message probably includes that info.)
echo "<pre>include_path: ".get_include_path()."</pre>";
Just a thought: if they include the version number in the name of the PEAR directory (or its parent directory?), you may just have to change the include path to reflect that name change.