sane-service-type seems to only support the minimal backends package:

--8<---------------cut here---------------start------------->8---
(define sane-service-type
  (service-type
   (name 'sane)
   (description
    "This service provides access to scanners @i{via}
@uref{http://www.sane-project.org, SANE} by installing the necessary udev
rules.")
   (default-value sane-backends-minimal)
   (extensions
    (list (service-extension udev-service-type list)
          (service-extension account-service-type
                             (const %sane-accounts))))))
--8<---------------cut here---------------end--------------->8---

The value of the service is a single package, not a list of packages.
So how does one add support for other scanner?

-- 
Pierre Neidhardt
https://ambrevar.xyz/