groupwise registration with labels

Hello I have dataset with 5 MRI images per patient plus label associated with one of them

It seems that groupwise registration could be best option for me like in for example [1], but I can not find a way to perform groupwise registration and mark to which image the label is associated - or to get the transformation field of this particular image in groupwise registration.
I see the possible answer in [2] but it is not completely clear for me about stacking labels in 4d- does it mean that for example when I would add images

['t2w', 'sag', 'cor'] 

Then get transformation maps by

transformParameterMapVector = SimpleElastix.GetTransformParameterMap()

for case that I have labels only related with t2w I need to in this case:
1)create 2 empty zero arrays of the same shape as labels
2)concatenete them in 4d array keeping true labels at the same position as t2w in inputs
4)retrieve label array from 4d array ?

Do I understand it correctly?
Any help will be highly appriciated !

[1] Groupwise Registration — SimpleElastix 0.1 documentation
[2] How to apply the transformation of groupwise registration on images and their corresponding label files? · Issue #255 · SuperElastix/SimpleElastix · GitHub

I think this question was better asked on Elastix group. But @Niels_Dekker might answer it, or better direct you.

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