In the US BMW would tow the car (or actually their roadside assistance contrator would) even if it was 500kms. Check, maybe they will in AU as well. But I would agree, an extra tire might be a good idea for those times you're far away, since you'd still in theory want the tyre to match anyway, etc. A steel rim could greatly lower the cost, or a 15" with the same outside diamater could be quite reasonable. Though its never simple.
Of course to calm your pessimism, you don't need to replace the tyres with runflats, so if you're stuck somewhere, if they have something that's 17" and basically the same size, it'd work easily in a pinch, and there's a reasonable possibility they could have the right size, just not in runflat. I suspect they'd be much better at removing a runflat then putting a new one on too.