-- Last Updated: Mar-20-13 9:11 AM EST --
This is an ultra-light hull, so even 20 pounds of pull applied at a right angle to any flat or nearly-flat portion of the hull will deflect it almost an inch, so I figured that the extreme amount of pull exerted by thigh straps and the like would be pretty much out of the question, before even considering the fact that I've never seen ultra-light hulls used in water that makes it necessary for such outfitting. Though the flat, un-reinforced parts of the hull are very flexy under very light load, the curved areas are much stronger and the the margin of the core is stiffest of all. I suppose you could cause a break at the joint if you stressed it enough, but that amount of force applied somewhere else would be destructive too, and I can't imagine that happening in flat water. The stress applied by float bags to a boat that's simply swamped (not pinned against a rock in current) is pretty negligible when spread across many connection points. After all, the only thing the straps which connect to the stems and bottom of the hull do in that case is keep the bag from slipping out from under the cage, and in a flatwater situation that doesn't take much. Tying in gear in a flatwater situation isn't much different.
And here's a thing about the kind of stress riser that you describe which you are not considering, and it's something that's well illustrated by the practice of welding stiffening members such as angle iron to sheet metal. It's easy to create a stress riser at the edge of the weld which attaches the stiffening member (analogous to the one you refer to where one edge of fabric comes to an end a couple inches away from the foam core), but in that case the stress riser is irrelevant. It would only be a problem if one could cause a very strong and isolated flexing stress crosswise to the stiffening member, such as by cutting out a section and stressing it in that direction. In the complete structure, the stiffening member functions to spread the load a great distance longitudinally away from the point of load, such that the amount of flexing stress in the cross-wise direction at any given location remains small (of course, in such a situation it's best to orient the member in the most effective direction for the type of stiffening that is needed, but the idea remains the same in all situations). You can illustrate this for yourself on any ultra-light canoe. Set it upside-down on sawhorses and push as hard as you can on the margin of the foam core. The only flexing you will see is in the sidewalls, where the force is transferred to the sawhorses through that very weak area of the hull. Any flexing across that foam margin will be too small to observe. On the other hand, bending any non-reinforced, non-curved portion of the hull as much as two, sometimes even three inches can be done very easily just by pushing with your thumb (pushing really hard will deflect the material even more). If it were me, I'd put my anchor points where the hull is stiffest, not most flexible.