Well, it's probably worth doing for the sake of your learning. However, if you are going to get into the source, I think it's highly likely that you are going to see that the driver provides such a feature to userspace code through means of an ioctl, and in that case, you will probably be able to set the brightness programmatically without ever having to interfere with the driver code itself. So for that matter, the objective doesn't really fit with the complexity you're taking on. If what I'm saying is right, you could easily implement this entirely in userland code by writing a binary and then putting a script in the startup execution so that it calls your binary.
If you really want to hack the driver, another way to do it is to just put the brightness setting in the driver's init function, so that when the module is loaded it turns up the brightness. In that case, you *should* get what you want because the driver will be loaded at system boot.