An example of why arbitrary lists won't work for NICS
You can sum up all of the reasons why the so-called "terrorist watch list" or "no-fly" list cannot reliably be included in the NICS process with two words; those words are Olga Cox.
For example, Olga Cox, who is listed as "Instructor - Psychology" on the faculty roster of Orange Coast College in California, has openly branded Trump/Pence supporters as "terrorists." Not only that, but she is openly teaching this stuff to her students!
Who is to say that Olga Cox, or someone like her, would not be willing to arbitrarily include the name of anyone who supports Trump/Pence on any secret list — on any list that does not require due process or probable cause? To do so would deprive that person of a civil right guaranteed by the Constitution.
/fl