CONFIG_CEPH_FS can be built as a module,
CONFIG_BLK_DEV_RBD cannot, so if you want a kernel that includes these options, you'll need to create your own AMI. However, if you're really lucky, you may be able to find an AMI that already includes the changes--but I don't see any obvious ones off the bat.
There are some posts dealing with recompiling the kernel; for example, on Ubuntu 10.04, although you'll find similar guides all over the place with a Google search.
At the very least, you may want to make sure your newly recompiled AMI is freely available to others, and you may even want to contribute the name upstream to the Ceph FS guys--since its a distributed file system, operating on EC2 sounds like a pretty common use case for it. And I'm sure the next person that wants to test it will appreciate your contribution!