Not sure what the impact of this would be for environments with specific environment configurations / attributes etc, but I know I had to remove cookbook version constraints for environments in order to run chef-client in local-mode - it might be the same for chef-zero and so a safe bet to make this change.
From looking at the chef-zero code, updates to _default will fail: https://github.com/opscode/chef-zero/blob/master/lib/chef_zero/endpoints/environment_endpoint.rb#L20
My chef-repo has environments/_default.json and https://github.com/andrewgross/vagrant-chef-zero/blob/master/lib/vagrant-chef-zero/action/upload.rb#L81 tries to update _default environment.
Not sure what the impact of this would be for environments with specific environment configurations / attributes etc, but I know I had to remove cookbook version constraints for environments in order to run chef-client in local-mode - it might be the same for chef-zero and so a safe bet to make this change.