What it comes down to is that there really isn't a formula for soap that does better than others at lathering in "hard" water per se. Rather, it's just that more lather in general means more under those conditions as well. The soap that lathers better in "soft" water will also do so in "hard" water. It's like whoever's taller will have a head poking above the others when standing on level ground, and also when standing in a hole, though the hole makes everyone look shorter.
When you dissolve soap in water, the more calcium & magnesium there was in the water, the less of the soap dissolves. The way to overcome that is simply to dissolve more soap to make up for what's used up by the minerals in making lime soap. You'd think you could mix something else with the soap to take up those minerals, but that something else is going to take up space (and weight) in the bar that would've otherwise been taken up by soap, so you're not gaining anything unless you can replace that amount of soap with something that weighs less and reacts with the same amount of minerals. There are some chemicals like that, but the difference in their weight and that of the soap they replace is hardly worth it, and if you try using much of them, they'll change the character of the bar to make it less soaplike and more crumbly.
You might have noticed that lime soap is an anti-foam, for instance against bubble bath foam, so that unless bath water is completely "softened" first, soap defoams the bath. That's true, but only at high dilution of the foaming material. When it comes to a thick lather in a small amount of water, lime soap has hardly any foam-depressing effect at all. (This is true of anti-foams in general.) So it's not like you gain anything in terms of latherability by "softening" the water with anything else more than you do by "softening" it by using more soap.
The only other "solution" is to use something other than soap entirely--some foaming and detergent surfactant that doesn't react with the water minerals. The chemistry of making those is more difficult than kettle process soap (although the starting materials are similar) and has a lot less history, and I doubt there'll develop enough of a fan base for it to take off as a hobby.
