In short, no - because environment variables are userland state only, you can't interact with them using system calls, the kernel doesn't keep a "canonical" copy of them on behalf of the process. So the "environment" is part of libc, and "libc's way" of interacting with it at runtime "is the way".

From the syscall interface point of view ... you pass the initial env of a process when you exec(), and the kernel copies that to (userland) memory of the new process. The fact "default initialisation" can copy from the environment of the exec()'ing parent, or the fact that the kernel can "read" a process' env (see /proc/<PID>/environ) doesn't change this; the kernel needn't be "accommodating" all the possible and impossible ways how a user application may want to interact with that state there, if you mess-too-much with it, you get garbage. Sooo ... the portability wart is setenv(), because as far as the system is concerned... your "initial" env is passed to you when exec() is called, and any modification thereafter is your concern, your problem, but foremost, your choice. And choices come with taking responsibility for the ones you make.