A basic API, written in just 15 lines of code.
libversion is the first name for this API that came into my head :D/>

Download: pastebin get HyefnZPT libversion

Functions:

setVersion("CraftOS Custom Version")
Overrides os.version() to display your text.

getVersion()
Gets OS version.

setVersionToCraftOS("1.7")
Resets version to "CraftOS (Your CraftOS version here)"

Also there is a standalone utility - verutil.

pastebin get UvGcyQv5 verutil

I would appreciate criticism very much!