How can the OAuth client 'custom' field be edited once the OTK has been integrated with CA Developer Portal 3.x?
This is not currently possible. Once integrating with CA Developer portal you will lost the ability to make changes to the Client custom field.
Attempts to workaround this using the clientstore API will also fail.
The OTK client 'custom' field was introduced in OTK-3.3.01 (January 2016) release.