Looking on the web I have only found auto bed solutions with the use of some resistors to low the output voltage of the sensors, or with the use of a optcoupler.
Bu I have though "Can I use the output of the proximity to pilot a 12V relay?"
Well it can be done. In the file there is a simple diagram too.
The sensor 12V input are the same of the ramps. The signale out of the sensor pilot a 12V relay.
The Com pin of the relay is the S pin of the Z min, and the NO pin of the relay is the "-" pin of the z-min.
yeah I know that in my printer there is a lot of spave between the sensor and the hotend :D
I have to change that part. But it works. With just a relay and the sensor ;)
Here you can find a video
and here my post on my blog (sorry only italian... you can use google translator ^^)