I found another suitable signal: ‘power setpoint phase 1’
That I can use switch on a relay the smartbmv
That then switches on an 8kW boost charger (160A actually) above 2000W power setpoint. With the boost charger on AC out 2 the MPII then adds an variable additional charge current neatly regulating itself to stay within the 40A AC in (grid fuse) limits, and even prevents fuse overload when a house power (boiler for instance) turns on.