Someone, I think it was turbof1, asked if beam wings are excluded in 2017.
I had an idle few hours so I tried sketching out the rear geometries. It would appear that it might be possible to have a beam wing.
There are some slight oddities about the regs which may have caused the rule writers to "allow" this.
In the 2016 regs there is an explicit paragraph, 3.9.2 that essentially restricts geometry between the end plates and from the top of the diffuser to the top of the wings to two closed sections. That is the two wing sections.
However in 2017 the geometry is more complex. The top end plates at either side of the wing stop at 450 mm above the reference plane and a lower end plate is defined from the top of the diffuser to 450mm. I think this is done to allow space for the "brake duct" since the wing end plate is only 60-75 mm from the tyre and the brake ducts are 120mm wide.
This offset means that the simple box used before does not suffice. So they have defined the inner and outer face of the lower end plate, and a box to put the wing in but from 450 above the reference plane to the underside of the wing at 600 mm I can find no wording to restrict having geometry. There is a restriction in the centre area associated with the centre wing support and that might make supporting the inside of a beam wing tricky.
So I think there is scope for beam wing functionality. At the very least there is a 50mm wide section at each side where the lower endplate has to connect to the upper.
Apologies for a rather wordy post but I'm not quite sure how to illustrate this. Nor am I sure that my interpretation is right.