I don't think the reason is "fundamental."
Which is pretty much was I said in post #8:
I can't see any fundamental reason why it couldn't be done.
I think the reasons are economic and practical
A grid-tie-only inverter is designed to do one thing: push the max power capacity of a solar array into the grid via the customers electrical system. It's configured as a current source, not as a constant voltage generator. What stops the inverter from inverting is if the grid disappears or goes out of an acceptable voltage and frequency range.
The reason we want a grid-tie-only inverter to always output max power is because that's how the customer saves money on their electric bill. Anything that reduces that maximum raises the cost of solar per kWh and makes it harder to sell. For that reason grid-tie inverters are based around maximum-power-point-tracking, which is pretty integral to the inverter.
Wouldn't the maximum power be that which you can get from the PV array? It would be reasonable to assume that the inverter stage would be rated to handle this.
Now what you're proposing is to put a charge controller between the PV array and the inverter.
A voltage regulator would describe it better if there are no batteries to be charged.
That reduces the efficiency of the system and thus raises the cost per kWh exported. That's why it is only done in solar when battery storage is desired (for totally off-grid operation or emergency backup), because then it makes sense to connect the PV and batteries in parallel to the inverter at the same voltage.
What you're proposing is already standard for that situation, but not for applications without batteries.
A couple or three points in no particular order.
Yes, the additional circuitry, if required would increase the capital cost of the equipment a bit and also reduce overall conversion efficiency a bit.
On the additional cost of kWh produced, I suppose it would depend on the period over which you amortised that capital cost and the cost of the capital required. I don't know for sure but my guess is that the extra cost of the circuitry required would be relatively minor compared to the cost of PV panels. And I don't see how it would affect the price per kWh you get for what you export.
Efficiency loss? Well, for what we do there are three stages in the system I mentioned. A rectifier to get from the variable frequency and voltage to DC, a voltage regulator to get fixed voltage DC, and an inverter to produce fixed voltage and frequency AC. Generally, the losses are below one percent of the total system installation for all three combined. Different field, I know.
If you do have batteries I think you'd need some kind of regulator to match PV voltage with battery voltage rather than just connecting them in parallel to cope with the varying input conditions. I think that concurs with the point you made on that.
Obviously what I said above about you giving me a patentable idea was tongue-in-cheek, but it was not totally a joke. The situation that we are interested in is where the customer already has a generic backup generator on an ATS. We want the inverter to be able to positively get a signal when the grid goes out and the ATS switches. Based on that signal it would start inverting in a different mode that follows the load, safely, with no chance of backfeeding the other generator(s). If you think it's simple and easy to develop such a multi-mode inverter without adding more than, say, 10cents a watt to the cost of the inverter, then you should be developing that product, because there would be a market for it. The solar industry would love to be able to tell customers about such a product.
Our market is for bespoke projects rather than standard off-the-peg products that you might buy from an electrical distributor.
So back to my original point.
I can't see any fundamental reason why it couldn't be done.
And maybe it has.