Technically init does do that, but the variables only exist until login. Login clears the environment.
I'm not sure if that behavior is given in any standard though, so you should be better off using /proc/cmdline (via the getbootparam function or otherwise).
Bootlocal and -sync happen before login, so the variables are seen there:
Test command line was "quiet newcode=56".
But as mentioned, not sure how portable it is to rely on that behavior.
edit: Of course it has other downsides, like ignoring single words, and multiple bootcodes (blacklist=a blacklist=b).