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I wanted to ask the same question! Did you find any answer elsewhere? We need the source code of NeumannValue function and want to modify it.
not at all a beauty In[101]:= Clear[selfDT] selfDT[rk_Integer?Positive] := Block[{rg = Range[2 rk]}, SymmetrizedArray[ pos_ :> If[Signature[Join[pos, Complement[rg, pos]]] > 0, 1,(* else *) If[First[pos] == 1, 1, -1]] ...