Well, I too thought that motor would have an EGR valve, but it appears that it doesn't (on US models - I know the 500SELs we brought in and Federalized all had them). The US system with Lambda control and three-way catalyst I guess takes care of the NOX.
If your car is a US model then timing is your only hope. The effective mixture is controlled by the lambda system. Only base mixture is adjustable and it would only affect the cold running engine.
Unfortunately, you may have a catalyst problem. The small precats right at the exhaust manifold are the redux cats. They reduce (opposite of oxidize) NOX to N2 and O2. You should be able to pass, by retarding the timing significantly. It may not have any power but it will help passing a lot. Also try and do the test in the morning with the car as cool as possible (even better in the rain) don't run the A/C.
A thorough decarboning will reduce compression and make a big difference.
Bosch Master, ASE Master, L1
33 years MB technician