The total length of code written by PolarKernel, is some 24,000 lines long, while the original Q2A code, for comparison, is 36,000 codes long. Needless to say, these aren't minor patches.
All this code, does not come under the CC-BY-SA license, as that only holds for user-submitted contributions. Q2A's GPL license, as well, is very permissive, and there are hundreds of Q2A developers who make proprietary plugins and patches for Q2A, so as far as the legal issues go, there is no need for PolarKernel to make his code open-source.
Now, polarkernel is not an employee of the site whatsoever. His work is purely voluntary, unpaid, and an act of kindness; in fact, he pays for the hosting (which will start to cost, I believe, next month or the month after).
This means, that any decisions about the code, are completely his own. If he decides to sell some of the plugins tomorrow, as proprietary, it's completely his decision, because it's his code, and his work is unpaid and voluntary. Given that he's been even paying for hosting, he can always choose to make the code proprietary, and sell it, if he wishes to. The fact that he's giving the code freely to us, and even paying for hosting, should be enough, shouldn't it?
Personally, I would absolutely love it, if the PhysicsOverflow code one day became open-source, but the decision is always polarkernel's, and there are technical barriers to making it open-source (complicated installation instructions, code not commented out, code made to just work for PhysicsOverflow, etc.) according to polarkernel.
Also, I wouldn't be very happy with the idea of setting up "mirrors" at the moment, when the site is still so small.