Hi & welcome! This sounds like a fun project. I'd love to see a pic of the tub. And am not sure what Windsor blocks are? Concrete? If so, they may be a poor liner for a planting because of the high lime content, which can cause stunting &/or yellowing (chlorosis) for some plants.
Sedum & Sempervivum genera have members that can survive being outside all year in your area. Probably some evergreen shrubs, which are not my area @ all. My minis aren't big enough for plants like that but your tub sounds like a slightly larger scale. Can't wait to see/hear what you do with it!
