library has already loaded when you call setenv, so what you're saying doesn't work in most cases.
It seems to be a need to use poorly written libraries. You might consider fixing them instead.
This issue with that “interface” is the environment is process global. If the library is being loaded dynamically (specifically for some task) it would seem that the parameters are local to that task and should be taken by some reentrent init method. Alternatively, the process could be forked and environment set in the child without concern for thread safety or polluting the environment (think of the children!).