As you have observed this cannot be configured directly in Survey123. A potential workaround for Android: if this can be configured in Eos Tools Pro, perhaps you could use a Mock Location Provider workflow to apply your transformations to all apps. The links below provide more details on this workflow:
Thanks for your reply. I forgot to mention we are iOS only, so no solution there.
However the EOS Tools Pro app also exists on iOS and seems to have a datum shift configuration page:
I'll look into that shortly