Despite having read the opposite, it looks like IPv6 is disabled by default on the latest OSMC release (2015.06-1). I've tried adding a sysctl.d file to set /proc/sys/net/ipv6/conf/eth0/disable_ipv6 to 0, but this did not help. It worked only when I ran the command manually. By going through numerous forum posts I eventually found out that you need to use the connmanctl CLI tool:
(after having unpacked /usr/share/usb_modeswitch/configPack.tar.gz to a temporary directory)
Update 2016-01-19: back in 2014 I reported this to the maintainers of various "modeswitch databases", and it seems it's made it's way into the Debian/Ubuntu version at least, according to the changelog (version 20151101-1).