Sounds good. Keep in mind that the interface routes are only part of the story. The gateway routes may need corresponding metric values as well. A quick test shows that this should be possible; no "File exists" error! For example
# /sbin/ip route add default via 172.16.100.1 dev eth0 proto static metric 1
# /sbin/ip route add default via 172.16.100.1 dev wlan0 proto static metric 9
# /sbin/ip route | grep ^default
default via 172.16.100.1 dev eth0 proto static metric 1
default via 172.16.100.1 dev wlan0 proto static metric 9
Sounds good. Keep in mind that the interface routes are only part of the story. The gateway routes may need corresponding metric values as well. A quick test shows that this should be possible; no "File exists" error! For example
# /sbin/ip route add default via 172.16.100.1 dev eth0 proto static metric 1
# /sbin/ip route add default via 172.16.100.1 dev wlan0 proto static metric 9
# /sbin/ip route | grep ^default
default via 172.16.100.1 dev eth0 proto static metric 1
default via 172.16.100.1 dev wlan0 proto static metric 9