Answer: Growing hardy plants as container specimens year round can be a challenge. According to Container Gardening for Dummies by Bill Marken (ISBN 0-7645-5057-8), selected varieties of the following evergreens might be possible choices:
Chamaecyparis (zone 5), Juniper (zones 4 to 6), Pine (zones 3 to 5), Spruce (zone 3), Thuja occidentalis (zone 3 to 6.)
Special considerations would include making sure the soilmix is well drained and yet keep the soil consistently moist any time it is not frozen, provide them with full sun, and fertilize sparingly since you do not want them to outgrow their container too quickly.
Select dwarf or slow growing forms with a shape that looks good in your container and location and make sure your specific selection is hardy in your zone.
During the winter, it is best if the roots (ie the container) can be insulated enough so that they do not freeze. This can be achieved by sinking the container in the ground, or by trying to wrap it with improvised insulating material such as bubble wrap.
The plants may also dry excessively in a windy location, and they may suffer from heat reflecting off of paved or other hard surfaces. Providing a windbreak and relocating the container to an area where there is shade so that the plant stays at a more constant temperature can help.
To some extent, growing hardy plants in containers year round is an experiment to see what works in your particular location. In many instances gardeners will enjoy the plant in a container during spring and summer, then plant it permanently in the landscape come fall -- or plan on replacing it come spring if necessary.
I apologize for the delay in answering your question and -- Good luck with your project!
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