I just realised I built a bang-bang lead I can interface with a 12v signal over the inverter-combi relay.
For instance: closed when battery can accept charge, open when full to 20% discharged....
There's allsorts of things wrong with this solution but
I built it for genset simulation testing but if I install it on the inverter output instead of the input then it does GTIs too...as long as you put a plug on the GTI as all mine have.
...this is a handy thing allova sudden, I was debating whether it'd keep it around or not.
I heart anti-islanding, it's useful in all sorts of ways they never expected.
You can also use the GTI side of the anti-islanding relay as a photocell trigger.