This seems not to be a problem for rancheros 0.4.1. When I mount (manually) the disk onto a mount point on the OS, the container IS able to see it when I use -v (e.g. -v /data:/var/lib/mysql). When I do the same thing on my 0.3.3 box, the container is unable to see the mount. Clearly I need to move off 0.3.3 
Now… the remaining question is how to mount this second volume at boot time. Do I need to create a start.sh to mount it (as in the example for mounting an NFS disk)? Is there a better way?