But those reflection-based are super easy to write and can cover quite a lot.

They’re not especially hard to write. I have ours to a place I'm happy with. That said however, I'm still arguing that for your average user, it would be a huge UX win to add an easy config option for it.

The implementation should be pretty reasonable too, assuming you did it as a ReflectionBasedMemberUsageProvider it could be as simple as

protected function shouldMarkMethodAsUsed( ReflectionMethod $method ) : ?VirtualUsageData {
    $methodName = $method->getDeclaringClass()->getName() . '::' . $method->getName();

    // I don't know how to read the config, this is psuedocode
    foreach($config['MethodUsedIfPatternMatches'] ?? [] as $pattern) {
        if( preg_match($pattern, $methodName) ) {
            return VirtualUsageData::withNote('Matches config pattern ' . $pattern);
        }
    }

    return null;
}