Try setting up a VPN with both ends on the same subnet.
Sounds a wee bit contrived, I'm not sure I see the issue nor how it is
NAT related.
Doesn't your VPN have to have server and clients on the same subnet?
Several years ago, I used to travel a lot with my work. I'd find
myself in a hotel which used the same subnet as I did at home. This
meant I could not use my VPN. I have since moved my home network to
172.16.0.0, to avoid that problem. I have only once seen anyone use
anything in the 172 block.