I'm a huge believer in fighting link-rot as much as possible, so unless the US govt seized my domain for some reason, I would make sure all swombat.com links would still work.
SEO shouldn't be hurt (unless of course you are running the same site at both swomb.at and swombat.com), however with that said you won't get the seo boost that .com gives.
Another thought: swomb.at could be considered worse than say swombat.at as it is using the TLD to complete the word but I may be completely wrong in that case.
1- you own the .com domain for branding, people understand it etc
2- your site isn't actually a .com
Would SEO be hurt by doing something like this?