The Martin Mariner had extended engine nacelles that housed the weapons bays, could extended nacelles have been fitted to the Sunderland/Seaford to enable them to carry weapons or more fuel in them.
From what I can gather, not much was done about the wing on the Sunderland when it became the Seaford, would it have been possible to give the Seaford an extended wing housing more fuel or weapons cells?
How about retractable wing floats as used in some of the later R.5/39 proposals?
From British Secret Projects
By 4th December (1939) the Air Staff had proposed some modifications to produce a Mk.II Sunderland with an extra 500gal (2,273 lit) of internal fuel, changes to the hull forebody, redesigned bomb gear. Folding wingtip floats (which increased cruise speed by 4mph and added a little range and twin fins to reduce the tendency to swing on take off