Eu consegui depurar isso um pouco mais - mas ainda sem solução.
Se eu fizer o seguinte:
export SANE_DEBUG_PIXMA=255
scanimage -T
A saída do scanimage é a seguinte:
[sanei_debug] Setting debug level of dll to 0.
[sanei_debug] Setting debug level of pixma to 255.
[pixma] pixma is compiled with pthread support.
[pixma] pixma version 0.17.18
[pixma] Scanner model MP150(Canon PIXMA MP150) not found, giving up! MX420 series
[pixma] Scanner model MP170(Canon PIXMA MP170) not found, giving up! MX420 series
[pixma] Scanner model MP450(Canon PIXMA MP450) not found, giving up! MX420 series
[pixma] Scanner model MP500(Canon PIXMA MP500) not found, giving up! MX420 series
... Cortar muitos scanners
[pixma] Scanner model MX410(Canon PIXMA MX410) not found, giving up! MX420 series
[pixma] Scanner model found: Name MX420(Canon PIXMA MX420) matches MX420 series
[pixma] pixma_collect_devices() found Canon PIXMA MX420 at bjnp://192.168.1.5:8612
[pixma] pixma_find_scanners() found 1 devices
[pixma] Scanner model MP150(Canon PIXMA MP150) not found, giving up! MX420 series
[pixma] Scanner model MP170(Canon PIXMA MP170) not found, giving up! MX420 series
[pixma] Scanner model MP450(Canon PIXMA MP450) not found, giving up! MX420 series
[pixma] Scanner model MP500(Canon PIXMA MP500) not found, giving up! MX420 series
... Cortar muitos scanners
[pixma] Scanner model MX360(Canon PIXMA MX360) not found, giving up! MX420 series
[pixma] Scanner model MX410(Canon PIXMA MX410) not found, giving up! MX420 series
[pixma] Scanner model found: Name MX420(Canon PIXMA MX420) matches MX420 series
[pixma] pixma_collect_devices() found Canon PIXMA MX420 at bjnp://192.168.1.5:8612
[pixma] pixma_open(): Canon PIXMA MX420
[pixma] pixma_connect() failed EINVAL
[pixma] pixma_open() failed EINVAL
[pixma] pixma_close(): Canon PIXMA MX420
scanimage: open of device pixma:MX420_192.168.1.5 failed: Invalid argument
Eu achei a mensagem EINVAL interessante.