Supernatting to the same /18 network with multiple peers

Hello Community

And no, the title is not a typo.. i just invented the word.

We (ISP/Hosting) got this ridiculous request a few days ago. Our customer needs to access the same /18 subnet through multiple Site2Site IPSEC to different peers(cities).

The firm hosting the /18 subnet does not allow our customer direct access for some reason...
Because of GDPR they cannot use the same tunnel when they transfer data for each peer (city).

So our job is to make every peer(city) somehow accept that they need to NAT a /18 subnet for our customer through IPSEC... unless we come up with some other idea..

If you got any crazy ideas, they are very much appreciated :)