You can't choose which mapt you want to you use because you are cleared the Ils-z or the Ils-y. You must carry out the missed approach for the respective procedure you are cleared for.
I can't ever remember being cleared via the ILS-Z or ILS-Y approach at YSSY or YBBN, It's always just cleared the RWY xx ILS so how are you to know if it's the Z or Y?
Also the mapt for the LOC approaches would have to be the same point in space, just defined by different DME distances,
e.g.,
SY-34R ILS-Z mapt 1.5 IKN DME
SY-34R ILS-Y mapt 3.2 SY DME