Custom Fields over REST (Magento 2)

How can one bring in custom data on orders on a Magento 2 store? For example, a comment left by a customer, the subtotal, or any other field that might be associated to an order. These would be columns that show up in an order_ table, either default with Magento or perhaps created by an extension, such as ShipperHQ. Is there any plan to open up custom field functionality over REST?