I do this and it works! Tomatoes need calcium to achieve their full flavor potential, and calcium is often greatly lacking in our soils. 95% of a dry eggshell is calcium carbonate. When planting your tomato plants crush up (to powder) about 4 or 5 egg shells and put them in the bottom of the hole. Then plant your tomato on top. They'll provide calcium and prevent blossom end rot.
Enjoy a Pastel Color Theme of Geranium, Petunia and Lobelia and make sure you have supported well! These supports are anchored and flush with the houses siding, so take that into account when mounting window boxes, you want them to be sturdy!