The HistoricPlanItemInstanceQuery is missing methods to query for variables, such as variableValueEqualsIgnoreCase(name: String, value: String). Other historic queries have these methods implemented (see HistoricCaseInstanceQuery, HistoricTaskInstanceQuery, HistoricProcessInstanceQuery).
Also, the PlanItemInstanceQuery (non-historic) has functionality to query for variables. All of these seems to use the AbstractVariableQueryImpl, except for the HistoricPlanItemInstanceQuery (perhaps for a reason).
Will it be added in the future, or is there some workaround to add this functionality to an existing query?