It is a true unibody metal design. The answer from a cost and modding standpoint is "no way" and the only practical solution is a power pack.
BTW, it would seem HTC must be eating some healthy margin with this design strategy. The build seems too good and that usually comes at a cost. Wonder what their financial quarter results will be for the next two cycles. My education and work background is industrial technology and computer electronics. The M8 cost implications give my brain twitches. Not just the material cost, but the actual manufacturing process and design cost. Yikes.