The environment of today is that people are discussing whether to implement Mode S, and when, for which types of flights/aircraft. There is a reason for this: traffic density, layering of airspace, accurate position/altitude reporting etc. In this environment, would you really want an altimeter in your aircraft which can only be set accurately if the aircraft is at a known height (ie. on the ground)?
Me personally, I would not dare take an aircraft with such an altimeter outside the local circuit although, technically, if you set it to the airfield elevation on take-off, write down the reported airfield QNH and adjust the altimeter with 30 feet up or down every time the QNH changes one millibar, it should theoretically display more or less correct throughout the flight. (Which way to adjust, and how to handle the transition layer, requires an amount of thinking I'm not quite capable of right now.)
If you want to go for the "original panel" look, why not install a small, modern altimeter somewhere more or less out of sight (or hidden behind a cover or something that's easy to remove - others do that too), and use that to calibrate your pride and joy in-flight? You could even get a mountaineering altimeter or a handheld Garmin eTrex GPS with internal barometer (I think the eTrex Vista has one) and use that for calibration.
All this is separate from the legal side though.