In the Req Approval workflow, is it possible to set up a task that uses Accounting Unit for the Filter Key? The only option I see is Req Loc.
The filter keys don't have a value on their own. When you're working in a flow, before you reach the user action, you need to assign the name of your filter key to the oCatKey variable and the value of the filter to oCatValue.
So, in your example, you would need:
oCatKey="Acct Unit";
oCatValue=REQINFO_ACCT_UNIT;