I have been experimenting with a number of different probes, and returned to something I'd only gotten tangentally working - a BL touch sensor on the V2. Why the V2? Because nice, new V2 motherboards have a handy JP socket exposing Serial2. Older boards all the way back to original V1s have solder pads or through board vias you can put a header on, but the V2 came ready. So I built a little adapter to let me test it.

In short, there's a lead from the 12V input which runs to a buck converter, providing 5v for the BL-touch and powering the 5v side of a 5->3v level shifter. Power and Ground from Serial2 plus PA3 go to the low side of the 5->3v level shifter, and the rest of the wiring is pretty much standard bl-touch. 

Does it work? It does. It probes just fine, though it's not clear to me why the bl-touch can't be at an agle. There's probably a reason.