Enabling services.avahi did not help accessing the scanner via the network. The printer was already working. So it seems that avahi is not the missing part.
Yes, I have tried and it works. The scanner is detected as: epkowa:usb:001:007' is a Epson L355/L358 flatbed scanner.
Maybe there is something to do with the way sane is packaged on NixOS. I will try to investigate this.