You said viable, so I guess you mean production ready, which excludes still experimental or "in gestation" languages like rust or ATS, but they are indeed worth

You said viable, so I guess you mean production ready, which excludes still experimental or "in gestation" languages like rust or ATS, but they are indeed worth a look (note that ATS in its 1st incarnation targets C). I'd like to point out that there was a post several weeks ago on /r/haskell of someone having implemented a BSD kernel module. There's also MirageOS, an OCaml implemented framework providing all the services of an OS, thus letting people boot their apps in a VM very easily (that's the aim of the project afaik, given that it comes from the same lab than Xen. While not specifically tuned for OS development, they seem able to cope well with the task. Note that both languages also have a native SSL library development ongoing, which is a part of the MirageOS framework in the case of OCaml.