Mark,

Did you end up getting your scanning issue resolved? You may have seem my 
own scanning-related adventures over on help-guix [0] where I discovered 
that adding the udev rules from sane-backends was needed. The service 
definition that I used to add them is:

(simple-service 'sane-udev-rules udev-service-type (list sane-backends))

I found that looking at the output of `scanimage -L` from the 
sane-backends package (yes there are user-facing tools in there ☺) as both 
my user and root helpful.

Best,
Jack

[0] https://lists.gnu.org/archive/html/help-guix/2019-03/msg00071.html