Can you make use of the sine bar principle?
Clamp a long straight bar parallel to the X axis. This bar would serve as a fence. One edge of your plate should be machined straight to rest against this fence.
Now, the bottom corner of the plate sits on the table, and near the opposite edge, you prop it up on something to get the correct height (using trig to figure out the sides of the triangle). You could lathe some cylinders to lay in the T slots to serve as props. You would measure the effective height of the cylinders above the table with a height guage. It might take you a couple of good guesses to turn the cylinders the exact diameter required to prop the exact angle needed.
Use edge clamps or toe clamps to keep the face of the part clear for machining.
Face the part, then release the clamps and slide the part along the fence and reclamp, repeat the machining procedure.