The API for custom fields will be available to your site regardless of whether or not you allow the custom field to be viewed or edited in the Recurly UI.
You can read custom fields on an individual account with a GET call. This functionality is available beginning in API version 2.14.
You can write custom field data to an individual account with a PUT or POST call through the API.
<account> <custom_fields type="array"> <custom_field> <name>foo</name> <value>asdf</value> </custom_field> </custom_fields> </account>