another vote against solid mounts. (Mechanical engineer here as well.. for what its worth..)
There are certain engineering choices that you just don't do, unless you have a damn good reason and a ton of analysis and testing behind it. One of them is to rigidly mount a heavy vibrating mass to a critical structural part made from aluminum without any damping. You just don't do it unless you have a ton of testing, because if sh1t breaks, its BAAAD. you are not talking cracked exhaust or balky shifts.. you are talking broken chassis.
if you want to take more deflection out of it: go stiffer, add a rigid intermediate shell for the poly bushing to press into the motor mount, or even some how add additional mounting points... But the difference between no damping and a little damping is huge when it comes to vibration fatigue in aluminum.
Steel, no problem in most cases. Aluminum, bad.
as far as having data to back it up. Nobody will have it, because this is something that is against common engineering judgement. By the way when Lotus do vibration testing on the chassis, the suspension mounting points are mounted on bushings, not solid points.
Heck.. its not just the chassis you risk cracking, you also risk cracking your tranny case or even engine block depending whether there are any flaws in the casting.
the rear subframe is steel, not alum. so any solid drive train would mound to the steel sub frame, technically, it would be "very" easy to add re-enforcement to the subframe. my gut tells me you would probably get better results out of re-enforcing the sub frame and using very stiff mounts, rather bolting in solid drive train mounts....
fyi - most real race cars have solid mounts, mine does. there is nothing special about solid mounts, i see them all the time. engine and transaxle are bolted straight to the chassis. but to your point - certainly its designed that way, on the other hand - its not rocket science either. there are two issues at work, drivetrain movement, and drive train flex, once enough movement forces flex in the frame, you have flex, and I am going to guess the elise subframe flexes... a good bit... so that solid mounts will just eliminate drivetrain movement and introduce lots of flex.
now, sure, the subframe on the elise does bolt TO the alumn. chassis. but there is a good amount of mass and length of material there. never the less, for the OP, i am going with my suggestion above - very stiff mounts and a re-enforced subframe. i am not convinced on the elise, for reasons mentioned, would get the full benefit of solid mounts. but, with a well thought out approach, i don't see solid mounts being "wrong" at all
i am sure his tranny can take solid mounting (god, i would hope so!) no idea about the yamaha block...
here is a thought - what did Newey do on his elise race car? maybe a call to wirewheel for a peek into how that car is built?