I asked, because initially, the G'MIC plug-in was not really made to read something else than .png and .cimgz files.
At least, on Linux, it doesn't have this possibility. I assumed that most of the image data input for the plug-in would be provided by GIMP itself (and this is done through the GIMP plug-in API, so using memory buffers instead of files).
I've added .png and .cimgz support only for exceptional needs (as if a filter needs to retrieve a .png file from the network to work for instance). This is maybe a limitation (but not really serious IMHO), but this also reduces the number of external dependencies, and we all know here this is often a pain to deal with them

Of course, as the Linux version doesn't have these extra-libraries, I would suggest that no new filters added to G'MIC could use them (at least I'll never add filters that require it myself).
So, my opinion is that the plug-in should not have these extra features enabled (as it is done in the default Makefile provided along the source package). Having these is mandatory for the CLI version 'gmic' of the framework, but I think the plug-in can easily live without them (it has been like this since the beginning).