Also... it might be possible to do the shifter button mod without actually cutting the shifter wire. Peal back some of the insulation to expose the wire but don't cut it. Then solder the FSR wires to the shifter wire and tape it up. Doing that would leave the Di2 function unmodified but also control the power level. So if you pressed the left 'shift up' button it would still send the left shifter Di2 signal for "shift up" but also trip the power level up function. You can leave the Di2 left shift up and down buttons undefined in the Di2 software. Obviously that isn't all that useful (doing two things with one button) but it is less destructive should you want to reverse it - just unplug the FSR wire in the top tube.The "cut the Di2 wires coming from the shifter buttons" approach works because the Di2 shift buttons are just switches. You can't cut the actual Di2 cables in the top tube (or anywhere else) for this because they are a proprietary version of a CAN bus - its two wires carrying digital signals. Shimano would have to make a Di2 device that acted like a solenoid and they haven't done that. Wish they would - no one else can do it legitimately as Shimano's bus protocol is proprietary.
It should be possible to use wireless buttons (bluetooth or ANT+) that communicate with a wireless receiver that operates a solenoid inside the top tube. The solenoid would then have a wire similar to the FSR wire that goes to the TCU (easiest way is to cut the FSR shifter's wire off of it but that's also silly expensive). There are parts for all of this except the FSR shifter wire but it would be a project to get it going.