The idea of a Jet-A1 turbo-diesel is fine. But the real justification (around here anyway) is the difference in fuel price. Jet A1 is from 20-40 p per litre - Avgas is 100 p per litre more or less! It would make a massive difference to operating costs. Power/weight ratio is still an issue though, especially for a helicopter. The economics all change, of course, if the politicos start to change the tax regime.
If Frank lived/worked in the UK/Europe, and the main Robbo market was here it would have happened already......... (but if that was the case, the Robbos would never have had carburetors and carb heat either)