The rest api now supports sorting tasks by the value of variables: see:

AT:

• I can sort tasks by the value of multiple variables

==
Notes:
http://stage.docs.camunda.org/api-references/rest/#task-get-tasks-post

"sorting":
[{"sortBy": "processVariable",
"sortOrder": "desc",
"parameters": {
"variable": "orderId",
"type": "String"
}}]
}