We are trying to use Gift Batch API and Gift (Batch) endpoint for automating Luminate Online gift imports. In the process we are adding custom fields to the batch gifts, using the information from flat files and data warehouse.
While Gift (Batch) endpoint does not include custom fields, it is possible to use gift id returned in the response to POST Gift (Batch) to add custom fields in the next step. Unfortunately custom fields are not visible in Edit Batch Gifts option in the batch review, so we are flying blind until the batch is approved.
The ID of the Post Gift (Batch) returns is not a gift table system record id, but a BatchGift table system record ID, so you can not use that ID with the Post Gift custom field endpoint as that is for gifts in the gift table not the BatchGift table.
A new endpoint is being worked on that will allow you to add custom fields to gifts in the BatchGift table.