See "Tool pre-setter" in the CNC Dictionary:
http://www.thewarfields.com/MTCNCDictTangentoWork.htm
This is a really handy gizmo to have. They're not uncommon on pro CNC machines. You'll want a super sensitive switch so that your tool height is set accurately and repeatably.
I've also seen this done by closing a circuit between the spindle and a block that is insulated from the spindle/rest of the machine.
You can buy pre-setter switches that light with an LED fairly cheaply on eBay. They're pretty accurate and one of those might be just the ticket.
The
VB/macros for Mach 3 are out there, already developed by others I do believe. Done right, you'll have a completely automatic way of setting tool heights.
The other approach to the problem is to make sure every cutter has exactly the same offset. Tormach sells tooling that makes this easier.
Best,
BW