Blast from the past, well the Toyota HSD drivetrain to e-converison seems never happened. After all those years and millions of these hybrids sold (and god knows how much crashed/written off), strange and shall we say humbling..it will be a goer . the inverter could be hacked by replacing the control side , cheap in parts expensive in programing . as time goes buy we will be sharing / learning how to tune and write the software . I for instance am almost completely lost on programing , but it will come.
You are right that one, probably the most straightforward approach is just to use the stock inverter as any other IGBT power stage/module on the market and replace/hook it up with own driving pcb (DSP) based on ARM cortex, which are pretty advanced these days and available ready-made in many flavors, under $500, some even with motor self-learning software routines etc. The other approach, leaving the inverter box intact and just hacking it layer above in the hybrid system ECU was a dead end, since you depend on the other stuff/systems in the car, which must be either left in state of confusion (error codes) - dangerous, or software replicated (faked) and/or physically transplanted from the hybrid donor, which is a bit pointless.