Douglas,
The ADFA BTech is effectively a condensed BAero degree. Converting it is not difficult and the RAAF would allow you (if so desired) to complete this. You just have to argue your case.
The RAAF requires 10 years ROSO after pilots course graduation. If you incur further ROSO during this period, it can be served concurrently (meaning you don't have to serve more than 10 years). For example, if you were to succeed with your dream plan, the timeline would be as follows;
Graduate Pilots Course - 10 years to serve
Hawk conversion and IFC (18 months)- 8.5 years to serve
F/A-18 conversion (6 months) - 8 years to serve
Tour in F/A-18 SQN (3 years) - 5 years to serve
Test Pilot School (1 year)- 4 years to serve (incur ROSO however has no effect)
ARDU (3 years) - 1 year to serve
Return to SQN / Complete degree etc.
My point is, you can easily qualify as an F/A-18 test pilot in 10 years (I know of many guys that have).
As mentioned, you will face citizenship issues with the US. You can achieve a US green card via numerous means (search google). One of which is to serve on exchange with the US military and see active service (has happened to quite a few guys)
A few guys have mentioned that you focus on the now. Its very good advice as the road is hard and arduous, and you need to put effort into the now.
The reality is, it is exciting times in the RAAF with the prospect of some great capability coming online soon. If you join now, you will probably see the JSF into service (subject to a billion hurdles), the A330 is coming soon, AWACs, C17.
Good luck