it merely amounts to defining supernaturalism out of existence; it furthermore commits the fallacy of begging the question since it turns out that an argument which is supposed to show that supernaturalism is impossible depends on a premise which rules out supernaturalism from the outset

Arguments of the form "A, therefore B" are not invalidated by A being equivalent to B or, equivalently, by the converse "B, therefore A" being as valid. If you wish to invalidate the argument, either show A doesn't imply B (but you've already conceded it does) or critically assess A. Indeed, tell us: what is your definition of the supernatural? If you won't give one, stick with the one used here.

Jos commits, implicitly, a logical fallacy here, an argumentum ad ignorantiam: there is no evidence for x therefore x is false.

No I don't. "I don't believe X because X is unevidenced" is different from "I know from X being unevidenced that X is false". The burden of proof is on those who believe in the supernatural.

it isn't true that there is "no evidence" for the supernatural, rather one might say that the status of the evidence offered (arguments for religion, Biblical claims etc..) is contentious

Either A is evidence for B or it's not. If there is a contention over whether A is good evidence for B, that doesn't say anything about whether or not it is evidence for B. Whether it is or not is an epistemic matter which, whether or not it is a matter of consensus among human beings, is nonetheless a matter of fact. Now you've never posted on richarddawkins.net before this thread, so here's an interesting question: what example, if any, of "Supernatural claim X is well-evidenced by Y" do you personally think is true? One will do for discussion. As I said before, I prefer to talk about individual examples rather than assessing "the supernatural" without ever defining it.