What an interesting thread. I've recently been involved in the development of an aviation handheld GPS and have been pleasantly surprised by the results.
A modern GPS chipset (such as the sirfstar III) contains pretty much the same geoid correction data as many IFR GPS sets on the market. They get a fix quickly, correct the data so its reference is local mean sea level and most importantly, report the accuracy of the fix.
In the case of this device, we chose to put a little flag in the corner of the altimeter to represent the accuracy of the fix. Usually, in flight with no external antenna, the accuracy is within +/- 25ft. If it isn't, it's +/- 50ft. And this is after a short period of time for the maximum number of satellites to be acquired. Just knowing the accuracy of the information being presented is nice.
Of course this doesn't necessarily make it suitable in the context of the OP's question, since you want everybody to be using the same reference point. Let me know when you succeed in getting a bunch of pilots to all do the same thing
