I am planning to just use the "high side" of the transformer (120/240 set of windings) with the low side windings just capped off. That way I should be able to input 120V from a small portable generator and bump it to 120/240V split phase. I know, I know, those ones with wheels and handles cause all kinds of service phone calls!
But this is for myself, not for a customer. Lol. This is only for when my standby gen takes a dump on me.
In regards to the ONE, something that may be of interest to others is whether a battery will be needed, or if someone could start out in just a grid-sell setup and add their battery backup later. I believe you had said this was probably going to be do-able. Is that still looking to be possible? I'm sure there are others on here who this would be of interest to.
Also, I will 100% second your thoughts, Robin, on 200A transfer not being cheaper on the install! Something for everyone to keep in mind is that you should ALWAYS have a backup option. While many of us have absolutely great faith in a product by Midnite solar, we all know that any electronic piece of equipment can fail! Therefore you not only need a 200A "visible lockable" disconnect (NEC code). But also some way to bypass that inverter and get grid or gen power to your main panel! Remember, the integrated transfer switch is wholly dependent on being controlled by the inverter, so if the inverter is in an error state, it will likely not give you grid power passthrough either! That normally means either a 200A DPDT transfer switch $$$$$$$!!!, or you can do a sliding main to bus interlock, but generally that limits you to about 125A bus breaker for backup.
Along with the whole bypass, you will now need a second service disconnect, as you run into a very interesting scenario since the 200A disconnect is your PV disconnect. Now you need a separate 100-200A fused disconnect for your bypass!
Don't overlook the fact that I said fused! Both the 200A AND the 100-200A for bypass need to be fused as those are now your service disconnects! And you will need to revise existing grounding, as your bond will now be in the new service disconnect(s). Oh yes, that also means you need to separate out ground and neutral in the main panel..... more time/money! (Many main breaker panels have ground and neutral all run together on the same bars, and it takes a bit of time and effort to separate those out!)