I'm attempting to access the ETC by period for each assignment after clicking on the time tracking task in the Data Warehouse for reporting. I see that it is a virtual attribute but am unable to locate the attribute itself to see if the table and column are available. Is this information accessible through OData?
The information you're looking for is in table DWH_INV_ASSIGN_PERIOD_FACTS. You will be able to find there ETC, Actuals etc by period.
For more information on what table to query for what, we recommend checking out the report "Data Warehouse Schema" in Jaspersoft, this will include a list of DWH tables and descriptions you will be able to use for your reporting OData needs.