Under Team Strategy, if you are creating a custom Gameplan, please make it possible to select from the DDLB for Linebackers the option to "PassRush". This option exists for the Defensive Line, so I don't see why it wouldn't be available for the LB's - in the NFL, LB's like Von Miller or Lawrence Taylor are there to rush the passer above all else, even if they do sometimes drop back in coverage.
If this option is added, then at least when you select "Mixed". you'll know "Pass Rush" is in there.