I haven't looked to see if you are over on Freds TDIclub forum. I'm all for engine swaps and mods even if people ask why. A man has to have a hobby.
as for a TDi in an Elise, its been done, sorta. Google the Opel Eco Speedster. It shares its chassis and suspension with the Elise. Ive looked for a salvage Elise with damaged front and rear clams but they went for stupid money if the suspension wasn't destroyed. They still make great track rats and don't have to worry about damage I suppose. A friend here in Knoxville
has over 130k on his Elise and jokingly said I have first dibs when he sells it, but I think he will shoot for over 200k before replacing it as his daily driver. Its just too much fun