I've successfully installed Ubuntu 13.10 on a partition on an external hard drive.
Maybe, but you won't know until you can actually boot it. Not sure what 'rEFInd', some software to modify the Mac bootloader? There are some members here who use a Mac who might be able to help. You might give some specifics on what you did. I've never used a Mac but it you did install the Ubuntu bootloader to the / (root) partition of its filesystem, you would have to modify the Mac bootloader somehow to have it poiint to it. If you've done that, some specific steps on exactly what might help someone point you in the right direction.
Installing Ubuntu on an external in a case like yours would probably work better if you had installed the Ubuntu bootloader to the master boot record of the external. You can easily do that. You haven't given much information on what else if anything, is on the external and what your intentions are.