A while ago before the mtg came along I swapped one of the 7135’s for a 7805 voltage regulator. Same package size but just needed a jumper to feed the IC 5V from 3S liion. I still have the board. It worked running 3s XML’s from 3S liion. It’s not quite as simple as the Zener mod but has a wider input voltage range. It wouldn’t be very difficult to design a board to use it instead of the Zener. For 2-3 cells the 2981 is better as it works ~5-16V.
Here is a link to the post showing how to do the mod. Having thought about it for a bit I don’t think this would be a good fit for 3S liion but 3S lifepo4 might work if the voltage sag is great enough.