Why not instead create an module extension? This would allow you to create the customizations you need without modifying the kernel. You could create a copy of the shop module within an extension and then edit as needed the userregister view.
Yes, that is always a possible solution. But, the problem here is that we are on really tight budget (and schedule), and passing custom parameters from shop/userregister is the only modificiation on shop module needed.
So, it seems to me like a rather drastic solution for such a minor funcionality.
