Most people who play Rugby wear Football Boots now anyway.
Unless I had no other option, I certainly wouldn't wear proper Rugby Boots for playing Football, unless I changed the studs at the very least, because I doubt you'd be allowed to play with Metal studs. I worked in a Sports Shop for five years (And half of that time in the Footwear Department), and can't think of one valid reason that you'd wear Rugby Boots to play Football in.