I picked up quite a bit of room inside the triangle on my Trek frame by using a vise and clamps to flatten the downtube on the inside of the triangle surface with some extra squishing at the bottle screw nipples to make them flush. I supported the outside curve over a much greater surface area using wood, rubber, and leather welding gloves to be sure I didn't deform the curves of the tubing at the exterior of the triangle. On the inside I used smaller pieces of wood to both get it to a much flatter surface and not mark it up.
I was fitting a rectangular Ping battery into the triangle and moved one portion of the downtube over 1/4" to fit the pack's corner without affecting the structural integrity of the AL frame at all. How well this would work on a given frame would depend on the shape and diameter of the tube, so you'd have to evaluate it on a bike by bike basis, but it's likely that making a small indention at the nipple of a water bottle screw would be possible on almost any bike.
