Faré,
neither iolib nor osicat are the right place for temporary file handling operators like the one I proposed. Both these libraries are specifically targeted towards POSIX systems, so there is no chance that they will cooperate, say, with ABCL.
Setting the process umask is something that is completely unrelated to portably creating temporary files.
If this is not going into Alexandria, there are two likely other outcomes: Either the code goes into Postmodern, giving the next person the same problem (and I've not had this problem once, but many times), or it will end up in a trivial-temporary-file library that may include some conditional code for setting up the default temporary directory in a meaningful way. With Quicklisp, this would not hurt much, but it certainly feels a little silly to additional trivial-whatever libraries for basic functionality.
Your remarks regarding how this request represents a "poor way to promote Lisp as a serious language" are unwelcome, unconstructive and wrong. Thank you for not continuing on that path.
-Hans