Hi,

you can model roughly your ESD protection if you know the characteristic of the clamping devices, but to my mind most of the time this is used just to check the effect of the protection to the other circuits... i would say not very useful.
ESD protection devices are very dependent on the layout and some of them depend on the fab also (e.g. if one technology is fabricated in two fabs, you won't get the same performance  :) ).
If you modified an original ESD protection layout (most of the cases that is the case...) and you know the characteristic of the original one (from measurements), then it is possible to predict the behaviour of the ESD protectionm, but it is not a short story, you need experience...

for ESD protection simulation technology simulators are used , like ATLAS... they can deliver quite good results, most of the cases.

br