Our '90 300TE has the orthopaedic option on the driver's seat so I would be very surprised if it was not available for your W124 cabriolet. Given the up-market nature of the 560SEC and the fact that your 560SEL has the option I would again be surprised if it was not available for the SEC. I must admit that the orthopaedic option was not a commonly seen one. I had never seen it until we purchased our 300TE as a used car and I don't recall having seen another since. We don't use it much for short trips but it sure is nice on 12 hour interstate journeys.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.