Lefty,
That Input is quite a jewel!
That is quite what we're looking for!
The reason why we had our mind locked into an arduino-based solution is the design of the other accessories.
The motor drives a four-stage air compressor which is used for scuba.
And the accessories like drain-vents etc will be controlled by arduino. (which enables us to open the vents during start-up, so the load is somewhat lessened...)
Of course, one could simply use the switch-over relais that already exist, but honestly, that bang during the switch-over... Don't tell me it's good for any part of the unit... 
But if there's a unit that performs the soft-start for us, we can use the arduino to simply supply a digital input to that "external" unit.
I was not aware that such things exist, but then again, it is quite common for some things to be used on a daily-basis on some parts of this world, while the other part thinks about wizardry... 
Great, this really simplifies the design! 
Cheers from freezing switzerland,
Daniel