The main requirement to support this is that a RoCE or other RDMA API needs to be exposed inside the cloud VM. This requires (1) the physical boxes have RDMA (likely universal at this point), but also (2) the virtualized network adapter, e.g. AWS ENA, to expose an RDMA API, which is much harder.
AWS did not support any kind of RDMA when I looked into it last year. Azure does, but in my understanding this is only in their "supercomputer partition," which is not really a cloud environment.
I've heard that AWS is looking to write an ENA backend for GASNet (a communication library), which could perhaps (?!) lead to them exposing RDMA and other low-level NIC features.
I think the answer is, it depends. Far memory is only useful when the CPU isn't involved. Which probably means, the VMM underneath should support VM to VM memory access without trapping the call. I don't think that's something VMMs support today. In fact, they're actively building measures to defend against such an access.
If there's Remote DMA (RDMA) capable hardware (Infiniband or 10-gigabit Ethernet pci card) and a hypervisor that supports PCI-passthrough, then guest VMs can do RDMA. Not especially applicable for cloud providers trying to offer generic VPS' but possibly useful on the backend for managed services where the per-customer VM is not exposed to the customer (Eg AWS Redshift).
I think it should be technically possible inside your virtual network, if the cloud platform and network gear were to support it.