I don't think a grandfather clause, as you're explaining it, would really work. People try that with speeding tickets all the time (I wasn't the only one speeding! There were people going faster than me!). Also, a lot of the time, when a copyright holder goes after a zine, they'll go after all the zines they can find for their works.

With so much fanfic out there, it is interesting that there is still not a test case. Yes, fans do succumb to C&D's, but it is interesting that no copyright holder (and there are some vitriolic ones out there) has yet taken it beyond a C&D.
Well, unlike things like music downloading, fandom doesn't hurt the products. So the right holders aren't stupid enough to take it beyond C&Ds and fans, so far, haven't been stupid enough to not listen to the C&Ds.
That said, I still think we have a chance of winning if we do take it to court, and I think something like the OTW is a good group to do it, because they'll have the power of a large chunk of fandom behind them, rather than just being some disgruntled fan who decided not to give in, and tries to represent themselves.