I had to do some reading on this one. There are a lot of answers.
1- You should find a better recipe. Chocolate spread recipes I have used come in one of two styles.
The water in your recipe seems to be a problem. The nutella style that uses fat, sugar, and chocolate. These will not really dry out. Sometimes the oil will separate a little and will need to be mixed back in. I would group frostings in this group although, of course, they contain far more sugar.
See this recipe:
http://www.foodnetwork.com/recipes/gale-gand/chocolate-hazelnut-spread-with-crepes-recipe/index.html
Thick chocolate candy. AKA fudge. These contain water but the sugar and water are cooked together into a syrup so the water is not able to immediately dry out. They will crystallize if they are badly handled- many recipes contain a little corn syrup to prevent this. This does not seem to be what you are trying to make.
2- Store your concoction in a lidded container that isn't too much larger than your sauce. I use mason jars to store mine. Alternatively you can push plastic wrap down onto the surface of the spread.
3- Refrigeration. Recipes that don't contain dairy don't need to be refrigerated and in fact shouldn't be because it causes them to harden.
4- Adding water to melted chocolate is only a problem when dealing with chocolate that is dry and meant to solidify such as a molded chocolate. In such a case even a drop of water will cause seizing. In this case where there is already quite a bit of water in the recipe seizing is not a risk and you shouldn't have any problem mixing a tiny bit back in. Do not add water to the nutella-like spread recipes. They do not contain water and it will not mix in.
I can't tell from your description what is causing your sauce to harden- it could be drying out, separating, or crystallizing. Any of these will be solved by reheating and thoroughly stirring. Although water could be added to restore dried syrup I think you will get better results by switching to a recipe that uses fat in place of the water.
As you noticed, white chocolate doesn't have the same fat or solid makeup as regular chocolate, and in baking that will make a huge difference. The proportions of starch/binder (flour) to fat (butter & chocolate) to protein (eggs) are what make the difference between cake, brownie, fudge and everything in between.
White chocolate tends to melt faster than regular chocolate, so the baking time will be shorter, and there'll probably need to be extra flour & butter to make up the difference.
Go ahead and google for a basic recipe & then you should be able to make most of the same add-ins (nuts, extracts/flavorings) as any regular brownie recipe.
I would warn you to stay away from the "marble brownie" recipes though - they're really popular, but these are not two batters that play well with each other.
EDIT: Regular chocolate varies broadly in its fat makeup - different countries have diff regulations on what types of (natural) fats and (artificial) emulsifiers can be added to chocolate and chocolate-derived products. White chocolate has the same ingredient list as milk chocolate (w/o the cocoa solids), but again, the proportions and allowed/disallowed additives may be different. So you have to mind your brands/country-of-origin, as well as the "family" of chocolate.
Hershey's (US): http://www.thehersheycompany.com/nutrition-and-wellness/chocolate-101/types-of-chocolate.aspx
EU standards: http://europa.eu/legislation_summaries/consumers/product_labelling_and_packaging/l21122b_en.htm (If someone can find a more consumer-friendly explanation of EU chocolate labeling, that would be great.)
Best Answer
Lemon juice is still mostly ... water.
Water to molten chocolate (white or not - there is cocoa butter in either), unless in the form of cream and in a sufficient amount that completely changes texture (giving you a ganache), is generally known to do exactly what happened in your case - seize the chocolate.