giving a quick look, i agree. it looks like it might be helped by triangulating up to the main cross bar, transfer the forward loads into the top rail. the curved shape of the hard tail will also want to collapse in on itself. the tube is much stronger in compression than in flexion, in this configuration, the top tube of the tail intersects the axle on the side and not at the end, with all the force tending to bend the tube, and given that the tube is already bent, it'll be easier.
i'm probably not explaining this right, and haven't done the math, the bending strength of the tube might be sufficient, but i wouldn't risk it without triangulating the heck out of it. think of a steel bridge, that kind of structure is what you would need, and like i said before, you'll want to brace it to the top member, which will transfer the loads down to the front wheel (the top member takes pretty much all the weight of the bike in compression, when you think about it).
if you like, i could try sketch up and scan what i'm talking about...
(and i'm not an engineer, just an engineering technician, hoping to go back to school to get that bachelor's in the fall...)