I like the general idea, but would tweak it a bit.
we have the Z axis touch sensor, how about putting a 1/2" carriage bolt in the
chuck, hooking the sensor to it, and then manually drill some holes in known
places, put carriage bolts through them and wire them all to ground (or have a
clip you can hook to each one as the sled approaches it)
Then the machine can go to where it thinks the bolt should be, and do a spiral
search from there.
once it finds a bolt, it can then move around to find it from different
directions to figure out where the center of the bolt is. (back off until you no
longer have contact, approach slowly to get the most accurate position, cross
over the bolt head and approach it from the other direction, this then gives you
two points, move halfway between them and drop until you no longer have contact,
raise up until you make contact, keep going up until you loose contact, drop
until you make contact)
the sled should be able to ride up over the bolt heads, and then it will drop
down when they get to the center hole, so the sensing should be accurate. the
approach of getting clear and approaching slowly should work around any catching
of the two bolt heads against each other.