I'm not sure if it is the answer - but fitting gull wings (whether
inverted or not) means that the wing root joins the fuselage at approx
90 deg - therebye eliminating the need for a large, drag-producing
wing-to-fuselage fillet.

Uhhh no. The fillets were there to DECREASE drag.

You only need fillets on high or low-winged a/c

Fillets are used to smooth out airflow and thus decrease drag. The air
over a wing is moving at a higher velocity than the air over the
fuselage, and when the streams mix you get turbulence and drag. The
fillets work to counteract this interaction and the drag it causes.