Well... off the top of my head it sounds like your going to have to load the pending tasks for each process instance just to fetch this data. You're setting yourself up for very poor performance doing this. The data which you're wanting to show is contained at the task level but displaying it with a mix of process instance data.
It can be done though, but I might suggest being clever on your approach.
The ProcessInstanceDataGrid class is the component which displays the DataGrid and columns. The ProcessInstanceDataGridModel class builds the list of columns which to show for this grid.
var aCols:Array = ListView.getDataGridColumnsForTrackingProcess(session);
See the TrackingTaskAuditModel class for more columns relevant to Task data.
tmpColumns = new ArrayCollection(ListView.getListViewDataGridColumns(ListView.TYPE_TRACKING,
processInstance.processName, session));
Mixing these columns might get you what you're looking for.
