Hi Neil,
Saw your post over on mynissanleaf and knew that name was familiar so I came looking.
I haven't read the whole thing but noticed you talking about the prius transaxle. I was just reading some DIYelectriccar posts yesterday and came across this....
1991 MR2 + 2004 Prius Transaxle - DIY Electric Car Forums
Lots of info in there that's pretty in depth (I think) and above my current knowledge of conversions (looking for a near bolt/weld on in 5+ years) but it sounds promising and a good value vs a conversion kit.