Trying to modify Class Variables in and Extension - is it possible?

I’d like to develop an extension for Simple Commerce that will allow non-logged-in users to make purchases. Following the thought on this thread, I decided to write an extension for the simple_commerce_evaluate_ipn_response hook, but hit a roadblock when I tried to alter the $this->post variable in the class from which it was called.

So, I gave up and wrote a hack, documented in that thread.

I’d much prefer to write an extension than a hack, but I can’t see a way around that hurdle. If anyone has a thought on an alternative, I’d be glad to hear it!

Thanks for reading thus far.