The simple-scan extension in the corepure64 repo does not detect my network connected hp all-in-one, but the following workaround seems to fix things.
Using the cups and hplip extensions: $ hp-makeuri 192.168.1.170 [ip address of the all-in-one]
HP Linux Imaging and Printing System (ver. 3.13.11)
...
CUPS URI: hp:/net/Officejet_6700?ip=192.168.1.170
SANE URI: hpaio:/net/Officejet_6700?ip=192.168.1.170
HP Fax URI: hpfax:/net/Officejet_6700?ip=192.168.1.170
..and then: $ simple-scan hpaio:/net/Officejet_6700?ip=192.168.1.170
Note that, even though simple-scan Documents/Preferences/Scan source shows the correct address prior to closing the application and dconf org.gnome.simple-scan.selected-device shows the correct address after the application is closed, the next time simple-scan is run it will not find the scanner...