Will the omission of the stout result in less flavor or will it be okay without it?
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I also cannot have beer, so when I see it in a recipe I sub out wine or stock. Beef stock might be a good sub in this case. The recipe won't raste the same, but you do need the equivalent amount of liquid, and the stock will add good flavor. (or a combo of red wine and stock).
An equal amount of cold coffee and a teaspoon or more of unsweetened cocoa powder.
Green's makes a terrific gluten free dark ale with I think sorghum and buckwheat. If you can find it, I'd try that.