I've seen P. Grandiflora left out all winter, and come out of dormancy fine, so one or two of those could be a nice addition :)
A ceph may be possible, but definetly remove it in winter, the can take light freezes, but not -5 below. One thing you should know, D. capensis is Sub-tropical, it can go dormant, but it's best not to let them go through it, try something like a Filiformis, or Anglica, as they're temperate.
Good luck with you Mini-bog