There isn't any built-in support for TCL that's getting applied just because of the tag. The answer in the linked question is simply being highlighted using the
set output [exec man cmd-name]
Unfortunately, that's probably not good enough for properly highlighting all TCL code. You might try something like
lang-sh, but the result may not be perfect, either.
Stack Overflow uses Google's Code Prettify project for the syntax highlighting support. It is a separate project from the Stack Exchange sites, and maintained by mostly different people.
As I advised Lukas to do here, you will have to either submit a feature request to Prettify and wait for the TCL language to be implemented, or submit a style containing support for the language yourself. Once Prettify is updated, the latest version can be deployed across the network, and you'll be able to specify
lang-tcl hints as you please.