@McHenry If I'm understanding this correctly, Client Site A, B, n... will each have a server and all share multiple instances at OVH for DR. First off, I hope you give the person that's doing the networking for this a raise .
Second, are you and/or a team the sole manager of the infrastructure? If clients have their own IT team that assists/uses XO(A), this will be more difficult. For the sake of this solution, I'm going to assume that the client can use XO(A) to start VMs in case of a DR scenario.
If I were to build it out, I would build an XO host in OVH and utilize the XO-proxy and the Self Service feature. This way, you can register the XO Proxy to all of the client sites, and manage who can see what VMs, networking/VLANS, and storage.
If I'm wrong about an assumption or solution, let me know!