Approximating a "Not Connected" pin on an IC with a high value resistor
Figure 1. The LT6375.


  The way that I was thinking about simulating this is using a MOSFET to switch the pin onto a very high value (100M or so) resistor to ground. 


Why bother? - just leave your MOSFET switch at high impedance. There's no need to add in a high value resistance.

Your problem will be what type of MOSFET switch to use. When open-circuit they will have an alternating voltage on them and this will alternate above and below zero volts as the LT device uses a symmetrical supply. (See marked points on Figure 1.) If your switch doesn't block symmetrically when off and doesn't conduct symmetrically when on you will introduce distortion.

You will have to research dual-rail CMOS analog switches, I suspect. I'm only familiar with the CD4016-style analog switches but they only run on single power rails.