Cheers for the tip! I've been wanting to do this for a while, but didn't think of disabling the sensors entirely. For some reason initctl doesn't work when called from a shell script, but you can add an alias to shorten the command.

Add the following to ~/.profile:

alias om='devel-su -c "/sbin/initctl stop sensord > /dev/null 2>&1 || true && devel-su -c \"/opt/operamobile-labs/bin/operamobile-labs > /dev/null 2>&1\" - user && /sbin/initctl start sensord" && exit'

Save, restart the terminal, and type "om". Enter the root password, and opera will launch locked in that orientation. When you exit, it'll unlock again. It's a hacky solution (note the nested devel-su ), but it'll do for now!