The e-ticket is the obvious answer, although it shocks me that e-documents are accepted for almost any purpose - anyone with a computer can print himself up the most wonderful "e-ticket". It won't fool the computer at check-in, but no one else will know the difference.

It probably won't assuage OP's fears, but I don't recall ever being asked by NRT immigration for any kind of onward travel documention. Just my faulty memory? At SYD check-in, if the agent cares, s/he will probably be satisfied by pulling up the OP's ex-NRT booking and seeing that it's ticketed.