Strategies: Dealing with Bad NAT
Be prepared with customer documentation describing how to create 1:1 NAT between nodes requiring VPN access and the NAT devices.
Be prepared to assist customers by facilitating communication between customer and their service provider.
If customers will require access from many nodes behind a NAT gateway, consider LAN2LAN access.
Consider another VPN implementation that will allow NAT traversal? Is a UDP wrapped client available? Leverage your vendor!