Thanks for helping me.
Actually I had come up with that declaration :
(file-systems (cons* (file-system
(title 'label) ;; ntfs uuid requires conversion
(flags '()) ;; options such as ro come here
but reading the sources in guix/file-systems.scm, I see that is not an option in fact. But I see that besides ext and btrfs there is now support for luks encrypted partitions in the way, and that is great news for my other disks.
So, for the ntfs one, I guess I'll have to manually mount / unmount it as you explained. Actually, after installing the ntfs-3g package, it worked with this :
sudo ntfs-3g -o uid=1000 -o gid=998 /dev/sdb1 /home/arnaud/backup3/
The uid / gid bits were necessary to get read-write access ; it worked flawlessly in console but not in Gnome's file manager for some reason, but that didn't bother me as I rely on emacs dired instead.
As for the GUI, yes I'm using Gnome at the moment but I intend to go back to my usual stumpwm, as I have not managed to have guile-wm running (but on paper that would be awesome - without the capital of course !)