Will any sugar substitutes brown in a crumble topping?
As breadcrumbs in a savoury topping brown quite nicely (and so does unsweetened pastry) I should think you'd be able to find a mix that would work. 

The recipe would probably involve both fat and some sort of starchy ingredient(s). You might need to switch from oven to grill for a few minutes when it's cooked to get enough browning -- and getting just the right amount of browning might be harder, as some ingredients might burn quite suddenly. Of course, only browned sugar tastes like browned sugar (unless you can find a suitable caramel additive).

Perhaps a good place to start would be a "flapjack crumble" recipe, modified in a similar way to the many sugar-free flapjack recipes out there.  A crumble topping that's more like pastry crumbs should also do work but the texture is likely to be less forgiving.