If we implement a function based on the fallback version in that file you could add XDG_CAMERA_DIR to the user-dirs.dir config file (with a default value equal to the Nokia equivalent) so users could specify their own location if desired. The function would then load the file and return this value if present, if not use a hard-coded default location. If the function was added to libhildon as part of something like hildon-utils.c all hildon apps would have access. To cheat even more, you could just give the function the same name as the glib version and #define the Nokia values to the XDG equivalents.
On a side note, are there any updates that need adding to the Fremantle-GTK3 version (utils/userland wise)? I've still got to work out a few issues with the current state of libhildon3 and then check to see if I need to restore any features in the current hildon-desktop implementation. After that I'll look to working out the Devuan kinks.