So, that's almost certainly a no go. Balls.
So, next option is a powermeter that works with 3D+ cranks (or DA9000) that also somehow allows for smaller than 110bcd chainrings to be fitted.
I'm guessing Rotor INPower can't be retrofitted to my 3D+ cranks? Which means the cheapest option is an entire new powermeter crankset, which is clearly not that cheap an option. :S