Default [Q] Mutirom: how to update secondary rom to 4.4.4

I have Multirom installed on Nexus 7 2013. My primary is Android L Preview and secondary roms are a rooted 4.4.4. Ubuntu Touch and an unrooted 4.4.3. This 4.4.3 wants me to update to the latest build but as I understand it if I let it "install and restart" this will fail as I am running Mutirom with a modified kernel. I vaguely understand that it should be possible to flash the 4.4.4 system.img file but if any one could just confirm how to do this without messing up my nicely working Multirom setup I would be grateful. If there is a better way of updating then please say so if you have the time. Thanks.