We cannot satisfy every usecase, there will always be usecases that are not covered by the Query API.
For all use cases which are "over the line" to be included in the Query API we want to offer an SQL extension mechanism, if the resultset ranges over an activiti-entity (Task, Execution, Job, ...; In that case there are already mybatis mappings we can reference).
I think it would be best to add the possibility for the user to specify a custom SQL query:
.sql("SELECT * FROM ACT_TASK WHERE assignee = bernd OR assignee = daniel")
Details are currently discussed via email