Is there no 'halfway house' when it comes to subframe mounts? Something like polyurethane bush material or somesuch?
No, the poly bushes are dreadful and don't sort the issue.
All Minis from 1959 until the mid-70's had proper solidly mounted sub-frames. This is what gave the Mini its legendary steering response and great overall 'feel'.
The rubber mountings were, as I said earlier, just to help meet the new target-market of middle-aged women instead of the 'younger' element who were then moving on to cars like the Fiesta, Escort, etc.
The sub-frame needs to be bolted to the monocoque body-shell as Sir Alec always intended in order to take the suspension loads directly into the structure rather than into flexible mountings.
All you are doing when fitting solid steel mountings is putting the car back as it was intended to be and thus achieving the original 'feel' of the car.