In other words, whether or not you can read/write a Netsuite custom field through web services depend on the security/permission set to the user login via web services.
Apart from the custom field level security settings, it is also important to check if the web services user is granted permission to manage custom fields, in particular, record type. For example, in Item Record Type, there may have been some custom fields setup for your organization:-
To be able to retrieve/manage the custom fields setup for record type, Item via web services, ensure that the user’s role is granted with ‘Custom Item Field’ permission as well:-
As such, you may want to check the above before you start utilizing Netsuite custom fields for any of your integration work or development via web services.