I guess I'll expand on my earlier comment. If you can run in sandals, huaraches are probably the lightest and most comfortable option. The problem with sandals is that you need clean form, a bit of scuffing or twisting can really add up to a problem over 26 miles. As you tire, if your form breaks a bit, you can also start having problems. Next step up would be a truly minimal shoe, I like the Merrell Vapor Glove, but there are others and you really want to wear whatever fits you the best. The problem with both sandals and truly minimal shoes is that they offer no support while protecting the feet enough to allow poor form. Poor form will abuse your feet, and 26 miles of abuse is a lot. Next step up is shoes that offer some support, and I would again go for comfort over style or opinion. You want to have done some long runs in whatever shoe you're going to race.
I see the "occasionally run barefoot on grass" comment as a huge red flag. I don't think that anyone should run in minimal shoes if they can't run barefoot comfortably on pavement. There's too much risk of injury. Smooth concrete should be more comfortable than grass. It's like running on butter. If it's not, you need to fix that before shopping for shoes. As a wet finger in the air, I wouldn't dream of wearing minimal shoes for a marathon unless I could easily do ten miles of pavement without any shoes at all ...