I also wish it were possible, but unfortunately, it seems that we can change the definition (including choice / alias list) of a Published Parameter only when creating or modifying the workspace. I don't think it's possible to update those dynamically at runtime.
Takashi
If you are looking for how to select an item from the choice list for a published parameter at runtime, as Nic mentioned, you can create 2 workspaces to do that. I felt that you raised a question about how to update items of the list dynamically.
Takashi