Thanks Seth, I was really replying to the custom function not working for not_if block rather than deal with the specific code or registry key methods in Chef.
As MW pointed out here: Windows registry_key > only_if/not_if > registry_value_exists help
And mentioned here: https://github.com/chef/chef/issues/2733#issuecomment-209703920
The use of registry_value_exists? and registry_data_exists? can be confusing but I agree using existing Chef functionality is best.