In the federal government preselection is a prohibited practise, but
how do you prove preselection occurred and what are some recent
prominent examples of preselection in the USDA/Forest Service where a
position description was tailored to meet a specific person's resume
to deny employment to others?

I have to tell you trying to prove preselection has got to be one of
the hardest things in the world to prove.  Hiring officials will cover
their tracks so well that in the design of the job announcement (while
being written for a specific person) will be open just enough to
include some skill sets found in the general population of existing
government employees.  Thus they say the announcement was indeed open
to all.  But you and I know that this is far from the truth. 
Additionally, the CPO types will side with the hiring officials. 
Generally CPO types are a little less helpful than tits on a bore.
That being said, make friends with a hiring official, get the job,
play the game.  As for me... I always hated this process and I am
leaving government service.  Take Care and be careful.

As an old hog farm boy, I assure you that saying is  "...boar". :)