If you lower the preload it will give you a more softer ride, i'm not sure that it will lower the bike anymore than the fact that your weight will drop the bike down. This would affect the handling of the bike also. Not sure if you can drop the forks through the triple clamp or not, I know you can on crotchrockets, but never had any request to lower a cruiser.

You may think about getting some shorter shocks??? that may do it without affecting the handling. (just an idea, maybe someone else could give you more info. and brands)