Sorting on Workflow custom process variables.

Sorting is allowed when querying the workflow API

http://localhost:8080/alfresco/api/-default-/public/workflow/versions/1/processes?orderBy=variables/[variable_name]

as described in

http://dev.alfresco.com/resource/docs/java/org/alfresco/service/cmr/workflow/WorkflowTaskQuery.OrderBy.html

but seems that the list of variables that allow sorting is hardcoded on Activiti:

ActivitiWOrkflowEngine.java
on the method
private void orderQuery(TaskQuery taskQuery, OrderBy[] orderBy)

I have the requirement from a customer to allow sorting on custom variables.