If you're looking for a way to start off in 1st gear by setting a switch, rather than pulling the shifter down into Lo every time you stop, I have seen a few solutions over the years, but none that made me jump out and do it.
I wonder if that is what that switch on the SEC really is - a 1st-gear or 2nd- gear start option?
I understand the pre '86 W126 can be electrically rigged pretty easily, there is a thread herein that's only a couple months old about it.
The later 126 is not so easy as I understand it. One guy rigged a fairly complex electrical signal to trick the tranny into thinking the accelerator was floored, and it shifted quickly to 1st on takeoff. Another offered a modified valve body that was supposed to work, but he wanted pretty big bucks for it.
One guy said to just re-position a spring and ball in the valve body, but others said this would trash the tranny.
So, after 20 years, I never made a move, but still might if something good came along. One disadvantage I think might be that 1st gear engages the brake clutch (B3 I think) and this would put more abuse on this first-to-fail component in the 722 tranny.

Anyone got any updated info on this, all my reading on it is years old.

DG