I think you can get there with a bit less effort. Rather than totally rethinking the sugar in your brownies, you can do what many pastry chefs already do for improved shelf life, and substitute 10-20% of the sugar with invert syrup. Buy Trimoline or just make your own. It has high freezing point depression, reduces sugar crystalization, and keeps things moist. You can also make the brownies sweeter than you usually prefer, since we're less sensitive to sweet flavors when they're cold. Scott's advice to add additional cocoa makes sense as well.
Other than that, just pick a recipe that isn't very rich. Not a lot of eggs, not a lot of butter or other fat. And one that uses cocoa powder rather than melted chocolate. Which is all to say: brownies that will be boring on their own, but that will add some tasty browniness to the ice cream, and that will be less likely to turn into little bricks.
You can use corn syrup instead of invert syrup, but it's not as good. I don't even mention corn syrup in any of my ice cream articles.
