May I suggest "issuing " a reg number as opposed to allowing people to choose after these slots are filled? It will be a real hassle and its a little odd. I've never seen a registry do that.
2000 Mustang GT turbocharged
2012 Focus SE Hatch
2008 Lincoln MKX
SOLD:#013 2010 Focus SES, 2004 Mazda 6s and 2003 SVTF #1496