Well, the statement "if a 10Ω output impedance amp with 450Ω impedance cable looks the same as a 460Ω output impedance amp." looks mostly correcy, except for the part that impedance is a complex number. So, this holds true for resistive impedances. If this is what you want, then you can use a low-impedance cable and add a 450 Ohm resistor with it.
If you want a cable with 450 Ohm characteristic impedance (in the same way as, say, 50 Ohm coax), then it will probably not work because the "apparent" cable impedance is only "valid" at high frequencies. At sound frequencies you'd need many kilometers of wire, the length should be comparable to the wavelength. Like, hundreds of kilometers (but then it will have huge losses). Otherwise it will look just like a small-walue resistor.
PS I googled what "James Randi speaker wire" challenge is. I'm not sure what you want to achieve here. For sure you can distinguish a coat hanger and a shielded cable. Just use a siggen and oscilloscope, or something. I think what he asks for is to prove that an expensive cable is better than a hanger, that's different.