civicrm/civicrm-core — pull request #1581 by vivekarora (2013-09-03 13:33:19+00:00)
Title: CRM-13325 - Added Attributes 

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* CRM-13325: Custom field HTML should indicate group+field name
http://issues.civicrm.org/jira/browse/CRM-13325
I have tested using this.
cj('[data-crm-custom="TheCustomGroup:TheCustomField"]').val('new value')