Nothing is currently mounting the posix message queue fs.
I've added the following in /etc/init.d/devfs to the list of dev related fs to mount:
"mqueue /dev/mqueue 0755 ,defaults none" \
seems to work well enough, but I'm not sure what options and DAC settings should be the default for it.
This is fixed in commit f30eec0 which will be part of openrc-0.12.
Thanks for the report.