I believe http://go isn’t special cased, but instead (ab)uses DNS search domains - the server is actually something like go.corp.google.com, and all Google PCs are configured to use corp.google.com as a search domain

Unofficial TLDs work fine without being special cased (well, so long as the browser is being pointed at the appropriate DNS server which knows about these entries), it's when the TLD itself is used as a URL you have to enter "http://tld." instead of just typing "tld." but any sub like "example.tld." still works automatically.