UML on my Linode
Using the Direct Disk or Grub kernel options, you can boot any guest installed kernel.
The problem you may face, is that the UML kernel you provide could be missing KVM or virtio specific kernel config settings. This could prevent LISH, networking, block storage, or other features from working properly.
You'll want to use a kernel with these flags compiled in or enabled as modules: https://www.linode.com/docs/platform/kvm-reference/#custom-kernel-configuration
In practice, I can't think of any reason to use a UML kernel over a normal distribution supplied (cloud ready) kernel. You may want to use the Full-Virt option (rather than Para-Virt) if you do have a technical reason for using a UML kernel.