The issue with the MOU is that the company (QF Group) has always abused the use of it to fit their shifting needs.
For years there was extremely limited access to JQ A320 slots even when they were rapidly expanding. There was almost no access directly to A330 slots (that I am aware of), guys had to go to the 320 and then wait for an internal transfer. No access to 787 slots. A320 slots that were available would be limited to specific and undesirable bases while the other bases were quarantined for JQ pilots.
Sometimes these decisions were for the benefit of Qantas who did not want to lose their pilots. Sometimes these were decision made to quell the anger of JQ pilots who felt aggrieved by the MOU positions. And sure enough when it suited the Group financially to allow the movement to flow from QF to JQ to relieve the surplus, the taps were opened.
So despite what the MOU actually says, the company will do whatever it wants in terms of making vacancies available under the MOU. If they don't feel they can spare the crew, then don't expect any vacancies to be available to JQ guys. Given they are already losing a significant number of returning QF MOU guys, they may decided they can't lose anymore pilots.
It is a very unfair situation but given the agreement spans multiple employers, unions and pilot bodies, it is very hard to fight the misuse of the MOU.