I had Renault 16 front seats at some time. Great ones, but they suffered from lack of support on the area just behind the knees. That's typical for any non-R4 seat fitted on an R4, as they were designed for level-standing floorpans...
I have a pair of 5 TS seats on the Jogging right now (yes, Lobster, they look great, but completely out of place on a Jogging), these are not as bad as I was thinking and I have completed some long journeys without any discomfort.
I have also seen Peugeot 106 seats fitted (no clue what modifications were needed). I have also tried to fit early Twingo seats, with no success, as the rail heights are not symmetrical.