Rackspace can totally do custom images now, there's just not a convenient way to do it from the little Control Panel. See here: https://www.rackspace.com/blog/cloud-images-for-the-open-cloud-import-export-and-share-images/
Booting a NixOS xen image should not be difficult; we typically use qemu, but enabling domU support should work just out of the box for the most part, I think. If you look in the Nixpkgs source, under nixos/maintainers/scripts/gce/create-gce.sh, there is a script to build a Google Compute image via nix-build and uploading it.
Basically, we need a Nix expression similar to nixos/modules/virtualisation/google-compute-image.nix to build a NixOS image. I think this is pretty doable.