the rated load limit is mostly a suspension thing....
A while ago in the buy sell section "Porky" <rangeman1@verizon.net> had some custom rear springs made for the insight to increase the rear load by like 35% at the time he was charging like $250.00 for a set ... I dont' know if he has any left .... it is on my wish list but I haven't got a set yet myself .... I hope he still has them or can get them when I am ready to get a set .... He said in an email to me that is was less than an hour project if you knew how to do it and had the right tools if not you could take it to your honda dealer or some repair shop you trust in order for them to do it but that would cost you labor ...
Also Mike ( from
http://www.99mpg.com ) beefed up his rear suspension and now runs about 300 extra pounds of weight in the back plus himself and passanger....
And thier is a guy who added allot of rear weight when he converted his Insight to a PURE EV
http://ev.whitecape.org/insight/ .... In all cases... I think it has not been any issue for the frame and such... it is just the springs , suspension , shocks and that stuff.