100 calorie bread?

The simplest answer to the request for low calorie bread, and the one I have used for years, is to slice a loaf as thin as the requestor will tolerate.  You can calculate the total calories in the batch from the weight of the flour (use what was on the bag or guess somewhere between 3.4 and 3.7 calories per gram of flour), then weigh it when you are ready to slice and slice accordingly.  With care you can slice Danish rye to 1/8" without too much trouble; a fresh ciabatta will be a little thicker (though not necessarily much heavier).

If you don't mind making small sandwiches (about 3 inches in diameter) you might try baking some bread using "Muffin Top" baking pans.   I check the nuitrion label of the flour package to determine the calorie per cup ratio when it's critical but I typically judge that a cup of flour (4.5 ounces) is approx. 400 calories and go from there.

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According to http://www.traditionaloven.com/culinary-arts/flours/plain-flour/convert-gram-to-calories-kilocalories.html, 1 gram of white flour = 3.51 calories.

So, let's say you want to make 8 rolls at approximately 100 calories each.  800 calories/3.51 = 228 grams of flour.

Make a 225g flour/yeast/salt/water dough (allows 3 g for the yeast calories) and divide it into 8 rolls when the time comes.

Of course, one could be more precise about the yeast calories and this assumes that the calories available do not change with fermentation, baking, etc.