If we're talking metric tonnes (or I guess a metric tonne if we are to be pedantic) then I'd say you're probably off by about fifty kilograms, at least. Kat, Shizuka and Asami are all pletty slim, but Alison, Amy, Liz and Jamila are pretty decently sized, i.e. they're not stick figures. So we can easily infer the averaged-out weight per person is about 65 kilograms for the ladies. But MC? Come on... He's tall, he's buff, and he's clearly not lacking in any way, physically speaking. So I'd estimate his weight at around 90 kilograms, at the very least.Since you mentioned it, I would be quite interested in the design of the bed. The static load on this screen will be slightly under half a ton, that's already a damn good dimension. And I'm not talking about what kind of dynamic load it has to handle.
So with that bit of data in mind, 65 x 7 = 455 + 90 = 545. So yeah! Your estimate was pretty good overall, but I'd say we can easily infer the bed to be supporting more than half a tonne of concentrated sexiness in that picture. Oak, ebony and mahogany, move the fuck over. We're using ironwood for this sucker, I'm betting! And I mean the D&D type at that, not just the olneya kind.
(And yeah, I'm a fucking nerd. Deal with it, yo. )
-Mal