You could implement an R and a C with Verilog-A equations.
How are you thinking to "remember" the starting voltage?
Code:module vexpout(in, out, ref);
inout in, out, ref;
electrical in, out, ref;
electrical mid;
parameter real R = 1 from (0:inf);
parameter real C = 1p from (0:inf);
analog begin
I(in,mid) <+ V(in,mid) / R;
I(mid,ref) <+ C * ddt(V(mid,ref));
V(out,ref) <+ V(mid,ref); // output buffer
end
endmodule