I 'think' it should be an easy thing to add though I am unfamiliar with objC code. My thought is that you would only need to implement the bit that fires the message when the self control period ends.
You probably would not even need to add a checkbox to the preferences if you didn't want to, since you could disable the notifications from the Growl panel
I'm taking a stab at this issue as I'd like it as well, but I'm having a little trouble getting my locally complied version of SelfControl to run correctly. Still trying.
I guess nowadays a notification center notice should be enough. :)