You could do it for a TLD without a subdomain. The TLD "tt" does in fact have MX records. But it's a very rare case that a TLD does, and prohibited for any of the new TLDs.

For non-FQDN hostnames, it's kinda pointless, as you can only use that on your own domain. So if you were to do that, just make up your own pseudo-TLD like ".x" with an alternate DNS system. You could even have it point to the regular DNS system through CNAMEs.