It’s not an aspect of GCP that I’m super familiar with but you may be able to route a static address to the cloud instance running your LXD instance, then route that into the LXD instance. When moving to another cloud instance, you can then reconfigure the VPC to route the address to the new host.
Looking at their docs, they seem to have a cloud router which allows for BGP peering. If that works the way I’d hope it does, it should allow for the creation of a VPC using dynamic routing which can then use our current work on BGP advertisements of addresses and routes to dynamically move those routes around.