Well I have to admit that it is extremely difficult unless you are a JavaScript guru. I have done stuff like this before, but it is at the very limit of my abilities.
The JS that controls OPC, is located in /common_files/modules/One_Page_Checkout/ajax.checkout.js
Very tricky file to work with, you will be interested in "_callbackPaymentMethodListChanged" I believe. Let us know how you progress with this, I am very interested in this subject. Hopefully someone with more experience than me will weigh in on this as well. I'd like to know who wrote the OPC code, it is fairly brilliant.
