What a cool idea for planters. I have seen olive buckets planted with success and I recommend using the same natural  material used to line window boxes and hanging baskets in hot climates. Not moss but the thicker coconut fiber that you can purchase in rolled up sheets either online or at home center stores. As for plants in your brutal climate I think that sedums, succulents and cacti do best so the spiky agave or perhaps cordyline will have the right geometry to look good in the tall olive buckets. Then allow a cascade of dichondra 'Silver Falls' or sedum burrow tail to spill over the edge. You might also want to find plastic pots that fit into your olive buckets and grow plants in the plastic pots then just set them inside the decorative olive buckets perhaps sitting on a base of another pot turned upside down. Keep growing, Marianne Binetti