I absolutely understand your idea, and it would most likely be possible to implement. The Question is really about cost/usage.
Your last example - one unit taking AC-PV-Power, converting it to DC, just so another unit takes DC-Power, converting it to AC, to feedin on another phase… That is: “DC->AC; AC->DC; DC-AC” - 3 conversion of probably 90ish % so, overall 72% efficeny (or 28% loss of PV-Power) - just to act a little more Grid-Friendly?
Especially when you talk about the people who are concerned to loose some money in overload situations. Do you know what they will say?
28% less money? Fine, let the others do that, i’m sticking to feed in on whatever phase brings me the most money.
So, if that Option would be implemented, without any (external) motivation there would be very very few people utilizing it. If the provider would somehow encourage this, by paying dynamic compensation per phase based on where he wants users to feed in - then it could be a whole different story.
Also, the overall Setting would be limited to very explicit usecases. Nowadays, each provider / country has rules, which power is okay, and when a 3-phased inverter is required. In Germany, the limit is 4.6 kW for single-phased feedin. Or in other words: Every PV-System that is providing significant amounts of power [to the grid] has to be 3 phased.
So, if you want to make your system a bit more grid friendly - and NOT lose the mentioned 30% of PV-Power from your single-phased AC-Inverter: Just hook it up to the weakest phase physically. It is highly unlikely, that tomorrow another phase will be weakest, as changing inverter phases is not a daily quest of people 
Side-Node
That’s self designed, backend is a c# application, frontend is vanilla html + javascript. (I’m a software developer, I always have to re-write already existing software “from the scratch” because i’m unhappy with 3-4 details and think I can do that better 
)
Loving it, scaling in realtime as currents are changing. Perfect replacement for sheep counting, when trying to fall asleep.