Another possibility would be to define a "lowest common denominator" ( for instance "code on Perlmonks by default comes with the same license as perl itself", or "code on Perlmonks by default is in public domain"), then display a disclaimer about this at account creation ( "unless otherwise stated, published code will be licensed as follow").
I know, how do we manage code posted since 1999 by 50000 existing monks? :)