What percent of current Secret Space Program recruits have served the interlopers in this way (as SSP recruits) in past lives?

More than 90% of the recruits in the current cohort of living humans have served such a program in past lifetimes, and this may have been many, many, times through the generations. This is the source of the trait of warrior seen by many who have an affinity for joining the military or who enjoy a good fight in some other sphere of human endeavor. This is also a hallmark of individuals who are currently serving the Secret Space Program or have done so in past lifetimes when it might have been designated other labels. As you know, the very term Secret Space Program is a misnomer because it is not truly descriptive. A more accurate designation would be Mercenary Army Program.