I'm baking a spice cake, with a maple-cinnamon BC frosting (and my first edible image) for my colleagues at the International Printing Museum, and I'd like to draw off just enough batter from it to make one cupcake for the same young woman for whom I was experimenting with vegan shortbread cookies.

She's evidently allergic to dairy, and my remaining two sticks of Earth Balance are already spoken-for. I don't want to shell out for another box of Earth Balance, or a can of canned frosting, over one cupcake. What I do have on hand is more powdered sugar than I'd need for the cake (9x13 single layer, decorated and served in-pan), pure maple syrup (Vermont Fancy, Vermont A-Medium, and Vermont B), a few basic spices, and (if I can't get away with a fat-free frosting) Canola oil.

Is there anything simple that would work with what's available?

The only options I've ever seen for dairy-free frosting were made with either shortening, dairy-free margarine, egg whites, or lard.

If eggs are OK, you could use a simple syrup-based topping, like this:
http://www.marthastewart.com/343280/seven-minute-frosting

Or just combine maple syrup with egg whites:
http://www.cooks.com/rec/view/0,1723,155172-232201,00.html

Found a recipe that consists of nothing but powdered sugar and water. Anybody have any experience with that? Its handling characteristics?

Would, something as simple as say, a teaspoon or two of maple syrup, thickened with powdered sugar, be my answer? Would it need something like cornstarch to keep it from being too sweet?

I'm not particularly interested in piling on enough frosting to equal the volume of cake. Just a thin coat. More like the thin, fully-set layer you find on a frosted donut.

The Martha Stewart recipe reminds me of simply boiling down maple syrup into maple cream, but that would be more trouble and expense than it's worth, and probably next to impossible for such a small amount anyway.

If you mix powdered sugar and water you will get the type of glaze on the top of a donut. But you can't whip it up thick like frosting. You can do all of this in a very small bowl or cup for just the one cupcake, with a couple of tablespoons of sugar to start with and drop by drop add water then the syrup, mixing with a spoon. Adjust the consistency as needed (add more PS if it's too runny, more water if too thick), and I wouldn't add cornstarch, since icing is meant to be sweet!

If you think powdered sugar and maple syrup is too sweet, add a teeny tiny bit of salt.

Went with a teaspoon of Vermont Grade B, with as much powdered sugar as it would hold and still remain fairly fluid, and a dash of cinnamon. Results can be seen in the last picture here:
http://cakecentral.com/cake-decorating-ftopict-748421.html