When I was in the states recently hour building the school there told me about multi jobs for small US charter ops. The deal is that you pay (they estimated 20USD per hour) to fly in the right hand seat logging turbo prop time and the airline benefits by not having to employ first officers. The school reckons that generally you get up to 100 hours a month this way and it would put you way ahead in the recruiting line of the other wannabees.
Also I dont think that visas are to much of a problem since technically you arent employed.
I have no idea of any names of these airlines but I am sure that a search of the net could yield something?