Since you mentioned you don't want to use any other framework, you are only leaving yourself one option: uopz
uopz is a black magic extension of the runkit-and-scary-stuff genre, intended to help with QA infrastructure.
uopz_flags is a function that can modify the flags of functions, methods and classes.
<?php
final class Test {}
/** ZEND_ACC_CLASS is defined as 0, just looks nicer ... **/
uopz_flags(Test::class, null, ZEND_ACC_CLASS);
$reflector = new ReflectionClass(Test::class);
var_dump($reflector->isFinal());
?>
Will yield
bool(false)
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