I was very surprised too when I recently found that international multi-city bookings are now "live" - but yes, it only seems to work on selected locations. For example, you can't book flights from one location to another within the US.
I was able to book as separate flights (on the one PNR) - PER-SYD, overnight stay then SYD-SEA (via LAX of course), then returning SEA-SYD, SYD-PER. Entering the final SYD-PER sector as an additional flight on the search (even though it was on the same day as arrival back into Sydney) gave me a better choice of connections SEA-LAX-SYD-PER than it did if I just selected SEA-PER.