Seaslug,
I hear what you're saying about most carts just not handling soft sand like we need when pulling a 70lb kayak across it. Have you seen or tried these Hobie Trax wheels? I heard they're a new item Hobie is selling.
They would utilize the scupper holes, but I believe Hobie's are reinforced with extra plastic around them. I haven't seen one of the Trax carts in person, so I don't know how wide the tires are compared to a Roleez cart, which has the widest low pressure tires I know of up to this point at least. By the tires in the photo though, they may be using roleez tires on them, or something like them at least. Roleez carts are nice, but I think they're way overpriced at about $170, and I've had to replace a set of bearings already because they cracked.
Roleez tires adapted to another make of cart:
Hope this helps,
Perry