Heinlein made it plain that it was the act of volunteering for service that qualified one for the status. The type of service was not the choice of the volunteer. That was chosen by the government according to the ability of the volunteer. If one were blind, deaf, and a paraplegic, they’d find something appropriate for that volunteer to do. . . perhaps mattress testing for the military. It still counted toward service and gaining the franchise.
People in service COULD NOT VOTE or hold office, contrary to what that idiotic movie depicted. Only after leaving service could one vote or run for office.
I’ve read the book.