Segregated witness soft fork: how is the funding Tx made "ANYONECANSPEND"
It's not prepending a script with OP_0. It's a data push that contains the witness program hash, prepended by OP_0.

Old nodes will evaluate this as a script that just pushes two data items onto the stack (a 0 and a hash). That's obviously spendable by all, as the requirement is having a non-zero item as last element on the stack.