question

Q 1. Using a BMS that can communicate a requested charge level (say in amps) to the Quattro via VE.Direct, can the Quattro be programmed to actively adjust its output frequency to control the power output of a grid-tied inverter (one that appropriately responds to frequency) to continuously achieve the requested charge level?

Yes the Quattro can change the frquency  3 diferant levels     ie  51 hz    then 53 hz  then 55    These levels can be set in the ESS  assistant

Q 2. So far I've only seen info that states the BMS can command the Quattro to start or stop charging. But I'm asking about active variable control of the charging current when in inverter mode, via frequency control.

The Inverter charger  controls whats being sent to the batteries,  the  PV  system supplys power to the quattro and if there is two much then the inverte increases the Hz  to tell the PV system to reduce the amount of power its sending,  so this will depend on the PV UNIT

Q 3. For reference, the BMS I have in mind is the Orion BMS2 by Ewert Energy. Its primary communications bus is CAN. Is there a straightforward way to couple between the CANbus and the VE.Bus or MODBUS of the Quattro? I think that is what the CCGX is for? Or better yet the Venus GX looks like it does this?

I dont nknow the Orion BMS  so you will have to work that out yourself.

Q 4. The system being designed will have 12KW of 60-cell PV panels, 48 KWHr of lithium battery, twin Quattro inverters (5KW to 10KW each; for split phase 240/120 it appears one must use two Quattros, 120V each, with their outputs in series; correct?), and either SMA or Fronius string inverters or possibly Chilicon microinverters. I believe all of these choices for AC-coupling the panel output can be set up to respond to frequency control.

I would stay  with the Fronius as there is more support with these units.  After saying that I am using a non standard unit and its working fine, but its upto me to set it up and its at my risk as well

At this point I don't necessarily need the details of HOW this would be programmed, but only WHETHER it can be done.

Thank you for your answers, Paul.