No reason not to. A lot of recipes do just that, or simply swell the gelatin in water before addition. However, and it's a big "however." This changes a recipe. You can't easily melt dry gelatin, so you'll need to compensate the additional water by removing it from someplace else. You take less chance on ruining a recipe if you try to take a balanced ratio of water from all the steps. Find a recipe that adds swelled gelatin or melted gelatin to get a feel for the numbers. Also, take notice of your timing. You say that you have to warm the whole thing up before you start counting, but that warm-up time is part of the ripening time of your recipe.