Won't cook. Will warm, though. To cook, you need some nice big reflectors to concentrate a lot of solar radiation onto the grill. BIG reflectors. And you have to shift them as the sun moves. So, not really practical though it might be entertaining to try it on land in a nice big backyard. You would want to keep the reflected heat in, by enclosing the whole thing in glass. It would take quite a cumbersome setup to cook at any reasonable temperatures. OTOH, a large cardboard box lined with mirrors or even aluminum
foil, including the top of the box left hinged to it, with a cover on the box itself, has been used to heat a black iron pot to crockpot temperatures.
You have to not only have a surface that will absorb solar heat, but also concentrate a lot of solar rays, and also prevent the escape of heat. And even so, results can be underwhelming unless you really go big.
I do think that the drying herbs idea is a good one, though.