I think it is absolutely true that a lot of young men who went into pioneering at the Vietnam era were primarily trying to avoid going to prison over the draft issue. It still was not a guarantee, but having a written record of service time was valuable documentation for a minister classification.
Indeed, it has long been my contention that the station of 'pioneer', which has no scriptural precedent, was purely an invention to to one thing, to give the brothers a way to claim they were 'fulltime ministers' and get their minister classification.