question

The simple answer is yes but that isn't actually a simple question, at all...

The Victron system, assuming it isn't permitted to export, or export enough to sink the AC Coupled load, needs to be able to limit the AC voltage, because the PV inverters will just keep pumping until the reach a high limit. I don't know, but I suspect, risking damage to the MP inverters.

Assuming you can talk to the PV inverters you 'could', in theory, hook them up without telling the Victron system that they were there, so it didn't try to export of shift the frequency, and then monitor the AC parameters, using them to decide how much solar input to accept. If your 'control system' stopped, for any reason, the AC voltage could go way up.

Another option would be a dump load, but that is even less of a good plan.

I am not recommending you do any of the above.

I expect the Victron frequency shifting is being done at a low level somewhere and will be pretty quick to respond to changes. It is highly unlikely that you could create a 'safe', in fact, fail-safe, system that would adequately replace that default frequency control.

I do not know how exactly the frequency shifting is implemented but even for me, a guy who lives outside the box, that particular plan sounds like a very bad idea and I wouldn't be looking at messing with it.

Is this grid linked, if so you could put the PV on the grid side so that the MP's can protect themselves and the PV would trip if the grid went down and the AC input switches in the MP's were open. You would want export limiting on the PV too, with metering points between them and the grid. There are options but it all depends on your goals and the capabilities of the specific kit.

Not a 5 minute task to plan, at least not to do right within equipment spec.

Why don't you want to just use the system that is available? just curious.

I realize this comment doesn't actually answer you question and I am hoping it prompts you to think about what you should be asking, even if it is only what you ask yourself.

Hey... Standing charges, absolutely.

Charging when not aware... It probably wouldn't, unless you looped output back to input to make it do things it shouldn't. Don't do that...

My point is that there is a mechanism, when off grid, that just works and is a supported solution, I don't understand why you would want to try and build a different solution to do the same thing, especially when the proposed solution is likely to be way worse than the one you can just switch on.

Almost nothing modern will care what the AC frequency is, within reasonable limits, why do you not want to use a well proven, and effective, built in solution to achieve you goals?

I am confused...

What you seem to want to do, works out of the box, please explain why you don't want to use interconnections/strategies/algorithms that do what you are asking for?

I have no issue with none standard, I have two MP's, a 24 and a 48, that are cascaded, not a solution I have seen anyone else attempt/do and definitely not something Victron would recommend.
it works because I haven't done anything to upset or threaten my kit...

Generally I think out of the box thinking is good, but you have to work within the limits of the kit you are using and draw a line.

What problem are you trying to solve?

Respectfully, I would suggest turning the problem/idea on its head...

If you were going to add CHP you would be better doing that by either having it charge the battery directly, which the Victron will not care about because it will just stop charging if the voltage go's up, or connect the CHP to AC In. The fundamental difference between CHP and AC coupled PV is that the CHP will just produce an AC voltage, a very stable one if you chose an inverter generator, and respond to demand by delivering what it can. There is no need to throttle it as it is throttled by default and responds to slight voltage drop by supplying current, just like nay supply/generator.

If you connected your solar to an additional inverter, in island mode, with its AC dedicated to AC coupled PV and then had that charge the battery it would also work, just like any external battery charger.

On the Victron you would, well could, see this as a DC system and any charge current, or load, that contributed to a change in SOC would be measured there. You would still probably need to do something in node red to manage the 'system', but electrically the battery is protected by the chargers and your code doesn't need to do anything critical or remotely fast.

Assuming that the CHP, and or inverter, is/are capable of being linked to an existing AC supply you could optionally choose to switch that on if demand was high and local resource was low.

I run two multi's, one feeding the second, AC out to AC in and have 2 batteries, it works just fine. The setup isn't by any means a supported solution and raised a few eyebrows when I proposed it but crucially it doesn't actually do anything that either system wasn't designed to do.
I have to manage AC input limits and charge current limits to orchestrate things to get the best/desired performance but I knew that going in and if my code, or coms. fail, for any reason nothing physical will break.