That's fantastic. Is that rule documented anywhere? (It could be quite important, so I'd like to be doubly sure

)
There's a long FT thread on the topic here[0], but the pertinent section is:
AA to/from AA or a oneworld® Carrier
If a customer is holding separate tickets on AA or another oneworld carrier, customers holding separate tickets where travel is on oneworld airlines should be treated as through ticketed passengers. In the event of a disruption on the originating ticket, the carrier responsible for the disruption will be required to reroute the customer to their final destination. The ticket stock of the second ticket must be of a oneworld carrier, eligible under the Endorsement Waiver Agreement. You may contact AA Reservations 1-800-433-7300 (U.S. and Canada) or outside the U.S. and Canada, reference Worldwide Reservations Numbers for additional information if the separate ticket is for travel on a oneworld carrier.
I understand that the AA page is no longer publicly accessible[1], but the Internet Archive has a version if you want something to print and carry with you[2]. Just close the Internet Archive banner at the top of the page before you print so that the page still looks fresh
Also, keep in mind that the although the standard minimum connection time at DFW for an International-to-International connection is 1hr10mins, it's actually 1hr20mins if you arrive on a Qantas operated flight. I'm confident that you're going to book a longer connection anyway, but keep that in mind in case you like to gamble. Details from EF below:
STANDARD.D/D...D/I...I/D...I/I.
ONLINE .50 .50 1.10 1.10
OFFLINE .50 .50 1.10 1.10
** OR * ARE ALL
QF-AA II 1.10 FLT 3000 - 4999 - FLT 1 - 5999
QF-AA II 1.20 ALL - FLT 1 - 5999
Happy travels!
p.s. If you ask very nicely at the Qantas check-in in SYD and provide your AA booking, they can even tag your bags all the way to CUN for you. You'll still have to clear customs in DFW even for International Transfers AFAIK, but it will save you from having to line up and get your bags re-checked in for the DFW-CUN flight.
Note - links below require you to manually remove the additional spaces (as I don't have 10 posts here yet and am not allowed to post links!)
[0] ww w.flyertalk .c om/forum/american-airlines-aadvantage-pre-consolidation/1283424-protection-irops-misconnects-separate-aa-oneworld-tickets.html
[1] ww w.aa .c om/i18n/agency/Booking_Ticketing/Reaccom/oneworld_tkt_policy.jsp
[2] we b.archive .or g/web/20150227235842/ht tp://ww w.aa.c om/i18n/agency/Booking_Ticketing/Reaccom/oneworld_tkt_policy.jsp