For a sample implementation focused on interfacing to the POSIX system and library calls, particularly illustrating how to use them (and also to make sure the SRFI was grounded in reality, the exercise was invaluable for that alone), I don't believe call-with-temporary-filename is important. It's pure Scheme using lower level SRFI 170 procedures in a certain way to achieve very useful results, it's one of a few higher level procedures that encapsulate important use cases for safe POSIX programming. I've got it partly done, cribbing some from scsh, but its requirements exceeded my current knowledge of Scheme, this was my project to return to Scheme programming in anger after many decades.

It would be nice to add the buffer-mode argument to those fdes->*-port procedures, but that would require me to dive way deeper into Chibi Scheme's port implementation code, I was able to do the other port work by following code that did much the same things as I needed. And as I remember this detail is also not POSIX, but Scheme implementation specific.

I would be happy to work with anyone who'd like to fill in those gaps to for example add testing (this being an interface to a side effecting API, the test code is anything but functional), but the whole implementation is not really production quality, or even consistently implemented as I (re-)leaned modern Scheme in the process. That's why I call it a "sample" one, making it solid as well as doing buffer-mode would require attention from a Chibi Scheme internals expert, for which there doesn't seem to be any interest (Chibi already had quite a bit of the functionality, which I freely cribbed).