I can see nothing in the documentation stating Island 60Hz requires N :/
I don't think firmware is documented. or debugged, or regression tested.
I do have an EGC I could tie to Neutral but isn't that essentially bonding G/N at the SB?
Nope, because you don't have a Neutral (wire)
All you have is a terminal labeled "N".
If the inverter drove current into "N", that would be objectionable current in ground wire.
But I think all Sunny Boy does is put a volt meter on N, to see L1-N and L2-N voltage and phase.
There are some "rules" which explicitly are listed L-L and L-N-L and I suspect all L-L would "work" in my case.
It would work if the firmware let it. It did for Rule-21.
The Island mode has different values for some parameters, and different functions (e.g. no anti-islanding).
Not all of SMA's parameters have been properly taken from settings, instead some were hard coded, and not all code has been properly enabled in various modes. I've found one bug, heard of others.
I have seen the jumper documentation and the only way I can see to set 240 without N is to setup a delta type connection but that wouldn't work in my case.
I had some wires with L1/L2/G from using a different model. For that I set jumpers for 240V.
A different connection I made for the less-old Sunny Boy I included L1/N/L2/G so left jumpers set for auto-select.
Try a wire connecting ground to the N terminal of Sunny Boy. I don't think that will behave any different from neutral. If you have a clamp ammeter you can check current in it.