Bob Pease is the man. In his book on analog circuitry, he suggests rather than measuring input impedance, measuring the input bias current. The equation he gives for an ordinary differential bipolar stage (with no emitter-degeneration resistors or internal bias-compensation circuitry) is 1/(20 x Ib) where Ib is the bias current . Now, one problem is that if you are actively seeking op amps with low bias current, wouldn't you expect them to have some sort of bias compensation circuitry?
 R.A. Pease, Troubleshooting Analog Circuits, Boston: Butterworth-Heinemann, 1991, p. 96.