This would require a pretty complex (and therefore expensive) mechanism, since there are somewhat significant forces being applied on the wasteboard during a cut (e.g. an upcut endmill pulling on the stock). The auto-leveling approach that Will mentioned achieves the same “perfect virtual flatness/levelness” by not moving anything but modulating the G-code values depending on the result of the surface probing.
There are probes out there that trigger upon mechanical contact, not electrical contact, so they don’t even require to cover the surface with something conductive. See this thread for example, quite interesting use of such a probe.