I was thinking of using two of these to charge two 4s batteries at the same time - my interest is in the lack of buttons or settings I would have to bother double checking, an overall smoother experience. I think these would be more enjoyable to bring out on the go - even better for compactness if I can repackage them with their power supply into a single package. At the moment I am using an accucel charger with a power brick that has long cables - useful for the home where wall plugs are in all sorts of weird places but for travel I much prefer something a bit less unwieldy.
I like what you are trying to do, but the idea has major flaws. Even with balance chargers, sometimes the cells get out of whack on their own. You need to be able to monitor this and make sure everything is within safe parameters otherwise you’ll end up with a puffy (and very dangerous) battery on your hands. The product that you have mentioned has no way to discharge either which means you would have to run your batteries down manually every time if you were to store them without use for a week or more.