My thumb nail also extends a bit past the north edge on the space and backspace keys, but I keep the back of my hands pretty low, so I hit those keys with the knuckle of my thumb, which finds itself in the middle of the keys. There are some fancy lubes out there, but I've had good results with WD40 and regular penetrating oil.
Some of the tried fancy lubes are:
Caig Deoxit F100 (oil used for soundmixer faders) which you can just put from the top without opening the switch
and
RO-59 tm KT (dry PTFE lube, you apply it wet, and it dries leaving a teflon film behind. Typically you would open up the switch to apply it... the Kinesis thumb switches can be easily opened up without even opening the keyboard case since they are PCB mounted)
This thread has some good info:
http://geekhack.org/showthread.php?22656-Gallons-of-Spooge-The-Official-Ripster-Lubricant-Experiment-Thread