I was able to add a z-axis limit/home switch this morning. Turned out that it was much easier that I thought once I got a switch and some wire/connectors.
I got a switch from a friend who does pinball repair and soldered some "extra" wire with individueal 0.1" connectors from an old computer to the NC (normally closed) and common pins of the switch. Plug in the connectors to the positions shown below (the order doesn't match as it's just a switch)
(like how the board can be removed with all wires attached? check out this link: viewtopic.php?f=106&t=3893
To position the switch I manually turned the z-axis down so it was about 1/8" from hitting the bottom with the printer in its normal upright position. I put hot glue on the back of the switch then positioned it against the side and moved up until the switch "clicked" open and held until the glue cooled. I then put extra glue on the ends to give a bit more strength
Now in Repetier you can use the z home button in manual control. When you click z-home the extruder will rapid move until the switch clicks open then will backup 1mm and do slow move until it clicks again.
The last step is to level the bed to your new home position. Apply whatever surface attachment you use (ie blue printer tape) then repeat the following steps for each corner
1) Click the motor off button to manually move the extruder to a corner
2) Click the z-home button then adjust the bed so there is a small paper-width gap between the end of the hotend and the bed.
3) Click z-home again to make sure the extruder doesn't touch the bed
BTW its much easier to adjust the bed height screw/nut if you move the bed left/right (with motor off), adjust the screw while holding the nut, then move back instead of trying to get a wrench in under the extruder while holding the nut
You can also edit your configuration to issue the z-home command by putting in "G28 Z0" in the printer configuration of slicr