As the title says I need to get some flights from Australia to Zaragoza in Spain.
I would like to use *A carriers as much as possible. Unfortunately ZAZ is not served by *A at all.
So far I have looked at SQ MEL-SIN-CDG-MAD and then connect with IB to ZAZ (about 4 hours transit in MAD)
or TG MEL-BKK-FCO-MAD then IB to ZAZ (about 3 hours transit in MAD)

Both thses work ok on the outbound but if I use the same trip return I will need to spend the night in MAD as the ZAZ-MAD flight arrives there late at night.
It would be possible to got ZAZ-MAD on IB and then connect to SQ FRA-SIN-MEL. Can one ticket something like that?

I like the SQ Spacebed anyway so that would be my No. 1 choice.

Is there anything I have overlooked? or is there a more convenient route there that I didn't think of?

You might be better off taking the train from Madrid to Zaragoza: it is less than 2 hours (high speed train) and services are frequent (roughly every hour or two hours depending the time of day) . Prices oneway are 43 Euros in tourist class, and 65 Euros in first.

If you definitely want to fly, whether you can combine IB ZAZ-MAD to a *A international itinerary will depend on the kind of fare you are on. On a full fare, this should not be a problem. On restricted fares, you have to look at the detailed conditions whether they allow an end-on-end with a non *A carrier.

I agree there is little reason to fly between MAD and ZAZ. Never been on RENFE's high speed services but have heard mostly good stuff about them. Depending on flight arrival time/train times, you could also take the train from BCN as it's still not much more than three hours and prices are a bit cheaper for some reason.
www.renfe.es

FYI, just read 5 minutes ago that SQ is dropping MAD come October.