PHP Dynamic Autoloader via extends - php

Question

Currently, I have a pretty average php auto loader loading in my classes. I've come to a point in development where I will need to override a class with another class based on a variable. I'm running a custom SaaS application, and we have the occasional organization that will demand some weird change to the way the system functions. In the past, we've filled up our code with garbage by massive IF statements for orgs, such as
if(ORGID == 'ABCD'){
//do this insane thing
}else{
//Normal code here.
}
So, I've been toying with the idea of a dynamic auto loader. ORGID is one of the very first defines in the application. The entire application is running under a fixed namespace of COMPANY\PRODUCT; Here's a code sample of what I was thinking I could do.
class MyLoader {
static public function load($name) {
$temp = explode('\\',$name);
$class = array_pop($temp);
$name = str_replace('_',DIRECTORY_SEPARATOR,$class);
if(file_exists(ROOT.'includes/_classes/' . $name . '.php')){
include(ROOT.'includes/_classes/' . $name . '.php');
}
}
}
spl_autoload_register(__NAMESPACE__ .'\MyLoader::load');
Since ROOT and ORGID are defined before the autoloader comes into play, I thought about doing this
class MyLoader {
static public function load($name) {
$temp = explode('\\',$name);
$class = array_pop($temp);
$name = str_replace('_',DIRECTORY_SEPARATOR,$class);
if(file_exists(ROOT.'includes/_classes/' . ORGID . '/' . $name . '.php')){
include(ROOT.'includes/_classes/' . ORGID . '/' . $name . '.php');
}elseif(file_exists(ROOT.'includes/_classes/' . $name . '.php')){
include(ROOT.'includes/_classes/' . $name . '.php');
}
}
}
spl_autoload_register(__NAMESPACE__ .'\MyLoader::load');
While this works, I have to copy/paste my entire class into the org specific class file, then make changes. I can't extend the primary class, because the classes share the same name. The only option I've been able to come up with that would allow me to extend my classes in such a way is to never load the base class.
Instead of
$myObj = new myClass();
I call
$myObj = new customMyClass();
and I have a file called customMyClass(); which simply extends myClass without making any changes to it. This way, the auto loader will load customMyClass and then load myClass. If an organization has their own customMyClass in their organization folder, then it will load in that class, which will then properly load in myClass.
While this works, we have hundreds of class files, which would double if we had a custom file for each.
I've seen a couple of examples that use eval to handle similar situations. Is that really the only way to do this type of thing?
UPDATE:
Just so I'm clear, the end goal is so that the thousands of places we've called $myObj = new myClass(); doesn't need to be rewritten.